CHAPTER 4

AIR MASSES AND FRONTS

Temperature, in the form of heating and cooling,plays a key roll in our atmosphere’s circulation.Heating and cooling is also the key in the formation ofvarious air masses. These air masses, because oftemperature contrast, ultimately result in the formationof frontal systems. The air masses and frontal systems,however, could not move significantly without theinterplay of low-pressure systems (cyclones).

Some regions of Earth have weak pressuregradients at times that allow for little air movement.Therefore, the air lying over these regions eventuallytakes on the certain characteristics of temperature andmoisture normal to that region. Ultimately, air masseswith these specific characteristics (warm, cold, moist,or dry) develop. Because of the existence of cyclonesand other factors aloft, these air masses are eventuallysubject to some movement that forces them together.When these air masses are forced together, frontsdevelop between them. The fronts are then broughttogether by the cyclones and airflow aloft. Thisproduces the classic complex frontal systems often seenon surface weather maps.

AIR MASSES

LEARNING OBJECTIVE: Determine theconditions necessary for the formation of airmasses and identify air mass source regions.

An air mass is a body of air extending over a largearea (usually 1,000 miles or more across). It isgenerally an area of high pressure that stagnates forseveral days where surface terrain varies little. Duringthis time, the air mass takes on characteristics of theunderlying surface. Properties of temperature, moisture(humidity), and lapse rate remain fairly homogeneousthroughout the air mass. Horizontal changes of theseproperties are usually very gradual.

CONDITIONS NECESSARY FOR AIR MASSFORMATION

Two primary factors are necessary to produce an airmass. First, a surface whose properties, essentiallytemperature and moisture, are relatively uniform (itmay be water, land, or a snow-covered area). Second, alarge divergent flow that tends to destroy temperature

contrasts and produces a homogeneous mass of air. Theenergy supplied to Earth’s surface from the Sun isdistributed to the air mass by convection, radiation, andconduction.

Another condition necessary for air mass formationis equilibrium between ground and air. This isestablished by a combination of the followingprocesses: (1) turbulent-convective transport of heatupward into the higher levels of the air; (2) cooling ofair by radiation loss of heat; and (3) transport of heat byevaporation and condensation processes.

The fastest and most effective process involved inestablishing equilibrium is the turbulent-convectivetransport of heat upwards. The slowest and leasteffective process is radiation.

During radiation and turbulent-convectiveprocesses, evaporation and condensation contribute inconserving the heat of the overlying air. This occursbecause the water vapor in the air allows radiation onlythrough transparent bands during radiational coolingand allows for the release of the latent heat ofcondensation during the turbulent-convectiveprocesses. Therefore, the tropical latitudes, because ofa higher moisture content in the air, rapidly form airmasses primarily through the upward transport of heatby the turbulent-convective process. The dryer polarregions slowly form air masses primarily because of theloss of heat through radiation. Since underlyingsurfaces are not uniform in thermal properties duringthe year and the distribution of land and water isunequal, specific or special summer and/or winter airmasses may be formed. The rate of air mass formationvaries more with the intensity of insolation.

EFFECTS OF CIRCULATION ON ALL AIRMASS FORMATION

There are three types of circulation over Earth.However, not all of these are favorable for air massdevelopment. They are as follows:

1. The anticyclonic systems. Anticyclonicsystems have stagnant or slow-moving air, whichallows time for air to adjust its heat and moisturecontent to that of the underlying surface. These

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anticyclones have a divergent airflow that spreads theproperties horizontally over a large area; turbulence andconvection distribute these properties vertically.Subsidence (downward motion), another property ofanticyclones, is favorable for lateral mixing, whichresults in horizontal or layer homogeneity.

Warm highs, such as the Bermuda and Pacifichighs, extend to great heights because of a lesserdensity gradient aloft and thereby produce an air massof relatively great vertical extent. Cold highs, such asthe Siberian high, are of moderate or shallow verticalextent and produce air masses of moderate or shallowheight.

2. Cyclonic systems. Cyclonic systems are notconducive to air mass formation because they arecharacterized by greater wind speeds than anticyclonicsystems. These wind speeds prevent cyclonic systemsfrom stabilizing. An exception is the stationary heatlow.

3. Belts of convergence. Belts of convergence arenormally not conducive to air mass formation sincethey have essentially the same properties as cyclonicsystems. However, there are two areas of convergencewhere air masses do form. These are the areas over thenorth Pacific, between Siberia and North America, andthe Atlantic, off the coast of Labrador and

Newfoundland. These two areas act as source regionsfor maritime polar air.

AIR MASS SOURCE REGIONS

The ideal condition for the production of an airmass is the stagnation of air over a uniform surface(water, land, or ice cap) of uniform temperature andhumidity. The length of time an air mass stagnates overits source region depends upon the surroundingpressures. From the surface up through the upper levels,such air acquires definite properties and characteristics.The resulting air mass becomes virtually homogeneousthroughout, and its properties become uniform at eachlevel. In the middle latitudes, the land and sea areaswith the associated steep latitudinal temperaturegradient are generally not homogeneous enough forsource regions. These areas act as transitional zones forair masses after they have left their source regions.

The source regions for the world’s air masses areshown in figure 4-1. Note the uniformity of theunderlying surfaces; also note the relatively uniformclimatic conditions in the various source regions, suchas the southern North Atlantic and Pacific Oceans formaritime tropical air and the deep interiors of NorthAmerica and Asia for continental polar air.

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SIBERIAN

SOURCE

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100 120 140 160 160160 180180 140 120 100 80 60 40 20 0 20 40 60 80 100

AG5f0401

ARCTIC AIRMASSES

AWINTER

EQUATORIAL AIR

CT

MA

MT

Figure 4-1.—Air mass source regions.

Characteristics of Air Masses

The characteristics of an air mass are acquired inthe source region, which is the surface area over whichthe air mass originates. The ideal source region has auniform surface (all land or all water), a uniformtemperature, and is an area in which air stagnates toform high-pressure systems. The properties(temperature and moisture content) an air massacquires in its source region are dependent upon anumber of factors—the time of year (winter orsummer), the nature of the underlying surface (whetherland, water, or ice covered), and the length of time itremains over its source region.

ARCTIC (A) AIR.—There is a permanenthigh-pressure area in the vicinity of the North Pole. Inthis region, a gentle flow of air over the polar ice fieldsallows an arctic air mass to form. This air mass ischaracteristically dry aloft and very cold and stable inthe lower altitudes.

ANTARCTIC (A) AIR.—Antarctica is a greatsource region for intensely cold air masses that havecontinental characteristics. Before the antarctic airreaches other land areas, it becomes modified and isproperly called maritime polar. The temperatures arecolder than in the arctic regions. Results of OperationDeepfreeze have revealed the coldest surfacetemperatures in the world to be in the Antarctic.

CONTINENTAL POLAR (cP) AIR.—Thecontinental polar source regions consist of all landareas dominated by the Canadian and Siberianhigh-pressure cells. In the winter, these regions arecovered by snow and ice. Because of the intense coldand the absence of water bodies, very little moisture istaken into the air in these regions. Note that the wordpolar, when applied to air mass designations, does notmean air at the poles (this area is covered by the wordsarctic and antarctic). Polar air is generally found inlatitudes between 40 and 60 degrees and is generallywarmer than arctic air. The air over northern and centralAsia are exceptions to this.

MARITIME POLAR (mP) AIR.—The maritimepolar source regions consist of the open unfrozen polarsea areas in the vicinity of 60° latitude, north and south.Such areas are sources of moisture for polar air masses;consequently, air masses forming over these regions aremoist, but the moisture is sharply limited by the coldtemperature.

CONTINENTAL TROPICAL (cT) AIR.—Thecontinental tropical source regions can be any

significant land areas lying in the tropical regions;generally these tropical regions are located betweenlatitudes 25°N and 25°S. The large land areas located inthese latitudes are usually desert regions (such as theSahara or Kalahari Deserts of Africa, the ArabianDesert, and the interior of Australia). The air over theseland areas is hot and dry.

MARITIME TROPICAL (mT) AIR.—Themaritime tropical source regions are the large zones ofopen tropical sea along the belt of the subtropicalanticyclones. High-pressure cells stagnate in theseareas most of the year. The air is warm because of thelow latitude and can hold considerable moisture.

EQUATORIAL (E) AIR.—The equatorial sourceregion is the area from about latitudes 10°N to 10°S. Itis essentially an oceanic belt that is extremely warmand that has a high moisture content. Convergence ofthe trade winds from both hemispheres and the intenseinsolation over this region causes lifting of the unstable,moist air to high levels. The weather associated withthese conditions is characterized by thunderstormsthroughout the year.

SUPERIOR (S) AIR.—Superior air is a high-levelair mass found over the south central United States.This air mass occasionally reaches the surface; becauseof subsidence effects, it is the warmest air mass onrecord in the North American continent in both seasons.

Southern Hemisphere Air Masses

Air masses encountered in the SouthernHemisphere differ little from their counterparts in theNorthern Hemisphere. Since the greater portion of theSouthern Hemisphere is oceanic, it is not surprising tofind maritime climates predominating in thathemisphere.

The two largest continents of the SouthernHemisphere (Africa and South America) both taperfrom the equatorial regions toward the South Pole andhave small land areas at high latitudes. Maritime polarair is the coldest air mass observed over the middlelatitudes of the Southern Hemisphere.

In the interior of Africa, South America, andAustralia, cT air occurs during the summer. Over theremainder of the Southern Hemisphere, thepredominating air masses are mP, mT, and E air. Thestructure of these air masses is almost identical withthose found in the Northern Hemisphere.

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AIR MASS CLASSIFICATION

LEARNING OBJECTIVE: Define air massclassification and describe how theclassification will change when characteristicsmodify.

Air masses are classified according to geographicsource region, moisture content, and thermodynamicprocess.

Geographic Origin

The geographical classification of air masses,which refers to the source region of the air mass,divides air masses into four basic categories: arctic orantarctic (A), polar (P), tropical (T), and equatorial (E).An additional geographical classification is thesuperior (5) air mass. The superior air mass is generallyfound aloft over the southwestern United States, but issometimes located at or near the surface.

Moisture Content

The arctic (A), polar (P), and tropical (T)classifications are further broken down by moisturecontent. An air mass is considered to be maritime (m) ifits source of origin is over an oceanic surface. If the air

mass originates over a land surface, it is consideredcontinental (c). Thus, a moist, maritime arctic air massis designated m; and a drier, continental arctic air massis designated c. Equatorial (E) air is found exclusivelyover the ocean surface in the vicinity of the equator andis designated neither c nor m but simply E.

Thermodynamic Process

The thermodynamic classification applies to therelative warmth or coldness of the air mass. A warm airmass (w) is warmer than the underlying surface; a coldair mass (k) is colder than the underlying surface. Forexample, a continental polar cold air mass over awarmer surface is classified as cPk. An mTwclassification indicates that the air mass is a maritimetropical warm air mass and overlays a cooler surface.

Air masses can usually be identified by the type ofclouds within them. Cold air masses usually showcumuliform clouds, whereas warm air masses containstratiform clouds. Sometimes, and with some airmasses, the thermodynamic classification may changefrom night to day. A particular air mass may show kcharacteristics during the day and w characteristics atnight and vice versa. The designators and descriptionsfor the classifications of air masses are listed in table4-1.

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Designator Description

cAk Continental arctic air that is colder than the surface over which it lies.

cAw Continental arctic air that is warmer than the surface over which it lies.

mAk Maritime arctic air that is colder than the surface over which it lies.

cPw Continental polar air that is warmer than the surface over which it is moving.

cPk Continental polar air that is colder than the surface over which it is moving.

mPw Maritime polar air that is warmer than the surface over which it is moving.

mPk Maritime polar air that is colder than the surface over which it is moving.

mTw Maritime tropical air that is warmer than the surface over which it is moving.

mTk Maritime tropical air that is colder than the surface over which it is moving.

cTw Continental tropical air that is warmer than the surface over which it is moving.

cTk Continental tropical air that is colder than the surface over which it is moving.

Ek Maritime equatorial air that is colder than the surface over which it is moving.

Ew Maritime equatorial air that is warmer than the surface over which it is moving.

S Superior air, found generally aloft over the southwestern United States, and occassionally at or nearthe surface.

Table 4-1.—Classification of Air Masses

AIR MASS MODIFICATION

When an air mass moves out of its source region, anumber of factors act upon the air mass to change itsproperties. These modifying influences do not occurseparately. For instance, in the passage of cold air overwarmer water surfaces, there is not only a release ofheat to the air, but also a release of some moisture.

As an air mass expands and slowly moves out of itssource region, it travels along a certain path. As an airmass leaves its source region, the first modifying factoris the type and condition of the surface over which theair travels. Here, the factors of surface temperature,moisture, and topography must be considered. The typeof trajectory, whether cyclonic or anticyclonic, also hasa bearing on its modification. The time interval sincethe air mass has been out of its source regiondetermines to a great extent the characteristics of the airmass. You must be aware of the five modifying factorsand the changes that take place once an air mass leavesits source region in order to integrate these changes intoyour analyses and briefings.

Surface Temperature

The difference in temperature between the surfaceand the air mass modifies not only the air temperature,but also the stability of the air mass. For example, if theair mass is warm and moves over a colder surface (suchas tropical air moving over colder water), the coldsurface cools the lower layers of the air mass and thestability of the air mass increases. This stability extendsto the upper layers in time, and condensation in theform of fog or low stratus normally occurs. (See fig.4-2.)

If the air mass moves over a surface that is warmer(such as continental polar air moving out from thecontinent in winter over warmer water), the warm waterheats the lower layers of the air mass, increasinginstability (decreasing in stability), and consequentlyspreading to higher layers. Figure 4-3 shows themovement of cP air over a warmer water surface inwinter.

The changes in stability of the air mass givevaluable indications of the cloud types that will form, as

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AIR TEMPERATURES

AIR TEMPERATURES

LIGHT WINDS

MOD. WINDS

STRATUS

WARM

WARM

COLD

COLD

BECOMINGOVER LAND OR WATER

BECOMINGOVER LAND OR WATER

FOG

AG5f0402

Figure 4-2.—Passage of warm air over colder surfaces.

COOL CONTINENT WARM OCEANAG5f0403

Figure 4-3.—Continental polar air moving from cool continent to warm ocean (winter).

well as the type of precipitation to be expected. Also,the increase or decrease in stability gives furtherindication of the lower layer turbulence and visibility.

Surface Moisture

An air mass may be modified in its moisturecontent by the addition of moisture as a result ofevaporation or by the removal of moisture as a result ofcondensation and precipitation. If the air mass ismoving over continental regions, the existence ofunfrozen bodies of water can greatly modify the airmass; in the case of an air mass moving from acontinent to an ocean, the modification can beconsiderable. In general (dependent upon thetemperature of the two surfaces), the movement over awater surface increases both the moisture content of thelower layers and the relative temperature near thesurface.

For example, the passage of cold air over a warmwater surface decreases the stability of the air withresultant vertical currents. The passage of warm, moistair over a cold surface increases the stability and couldresult in fog as the air is cooled and moisture is addedby evaporation.

Topography of Surface

The effect of topography is evident primarily in themountainous regions. The air mass is modified on thewindward side by the removal of moisture throughprecipitation with a decrease in stability; and, as the airdescends on the other side of the mountain, the stabilityincreases as the air becomes warmer and drier.

Trajectory

After an air mass has left its source region, thetrajectory it follows (whether cyclonic or anticyclonic)has a great effect on its stability. If the air follows acyclonic trajectory, its stability in the upper levels isdecreased; this instability is a reflection of cyclonicrelative vorticity. The stability of the lower layers is notgreatly affected by this process. On the other hand, ifthe trajectory is anticyclonic, its stability in the upperlevels is increased as a result of subsidence associatedwith anticyclonic relative vorticity.

Age

Although the age of an air mass in itself cannotmodify the air mass, it does determine (to a great

extent) the amount of modification that takes place. Forexample, an air mass that has recently moved from itssource region cannot have had time to become modifiedsignificantly. However, an air mass that has moved intoa new region and stagnated for some time is now oldand has lost many of its original characteristics.

Modifying Influences on Air Mass Stability

The stability of an air mass often determines thetype of clouds and weather associated with that airmass. The stability of an air mass can be changed byeither thermodynamic or mechanical means.

THERMODYNAMIC.—The thermodynamicinfluences are reflected in a loss or gain in heat and inthe addition or removal of moisture.

Heat Loss or Gain.—The air mass may lose heatby radiational cooling of Earth’s surface or by the airmass passing from a warm surface to a cold surface.The air mass may gain heat by solar heating of theground over which the air mass moves or by the airmass passing from a cold to a warm surface.

Moisture Increase or Decrease.—Moisture maybe added to the air mass by evaporation. One source ofevaporation may be the precipitation as it falls throughthe air; other sources may be a water surface, ice andsnow surface, or moist ground. Moisture may beremoved from the air mass by condensation andprecipitation.

MECHANICAL.—Mechanical influences on airmasses depend upon movement. The mechanicalprocess of lifting an air mass over elevation of land,over colder air masses, or to compensate for horizontalconvergence produces a change in an air mass.Turbulent mixing and the shearing action of wind alsocause air mass modifications. The sinking of air fromhigh elevations to relatively lower lands or from abovecolder air masses and the descent in subsidence andlateral spreading are also important mechanicalmodifiers of air masses.

The thermodynamic and mechanical influences onair mass stability are summarized in figure 4-4. Thefigure indicates the modifying process, what takesplace, and the resultant change in stability of the airmass. These processes do not occur independently;instead, two or more processes are usually in evidenceat the same time. Within any single air mass, theweather is controlled by the moisture content, stability,and the vertical movements of air.

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WORLD AIR MASSES

LEARNING OBJECTIVE: Describe thetrajectories and weather associated with worldair masses.

NORTH AMERICAN AIR MASSES,TRAJECTORIES, AND WEATHER(WINTER)

The shape and location of the North Americancontinent make it an ideal source region and also permitthe invasion of maritime air masses. You must be ableto identify these air masses and trace their trajectoriesto develop and present an in-depth weather briefing.

Within an air mass, weather is controlled primarilyby the moisture content of the air, the relationshipbetween surface temperature and air mass temperature,and terrain (upslope or downslope). Rising air is

cooled; descending air is warmed. Condensation takesplace when the air is cooled to its dew point. A cloudwarmed above the dew point temperature evaporatesand dissipates. Stability tends to increase if the surfacetemperature is lowered or if the temperature of the air athigher levels is increased while the surface temperatureremains the same. Stability tends to be reduced if thetemperature aloft is lowered. Smooth stratiform cloudsare associated with stable air, whereas turbulence,convective clouds, and thunderstorms are associatedwith unstable air.

cPk and cAk Air in Winter

The weather conditions with cPk and cAk air overthe United States depend primarily on the trajectory ofthe air mass after it leaves its source region.Trajectories, as observed on a surface chart, areindicated as one of the trajectories (A, B, C, D, E, F, G)

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1. Heating from below.

2. Cooling from below.

3. Addition of moisture.

4. Removal of moisture.

1. Turbulent mixing.

2. Sinking.

3. Lifting.

B. MECHANICAL

A. THERMODYNAMIC

Air mass passes from over a cold surface toa warm surface, or surface under air massis heated by sun.

Air mass passes from over a warm surface toa cold surface, OR radiational cooling ofsurface under air mass takes place.

By evaporation from water, ice, or snowsurfaces, or moist ground, or from rain-drops or other precipitation which fallsfrom overrunning saturated air currents.

By condensation and precipitation from theair mass.

Up- and down-draft.

Movement down from above colder air massesor descent from high elevations to low-lands, subsidence and lateral spreading.

Movement up over colder air masses or overelevations of land or to compensate forair at the same level converging.

Decrease in stability.

Increase in stability.

Decrease in stability.

Increase in stability.

Tends to result in athorough mixing ofthe air through thelayer where the tur-bulence exists.

Increases stability.

Decreases stability.

THE PROCESS HOW IT HAPPENS RESULTS

AG5f0404

Figure 4-4.—Air mass changes.

shown in figure 4-5. In the mid-latitudes, for an air massto be classified as arctic, the surface temperature isgenerally 0 degrees Fahrenheit (-18 degrees Celsius) orbelow.

TRAJECTORY PATHS A AND B(CYCLONIC).—Paths A and B (fig. 4-5) are usuallyindicative of a strong outbreak of cold air and surfacewinds of 15 knots or more. This wind helps to decrease

the stable conditions in the lower levels. If this modifiedair moves rapidly over rough terrain, the turbulenceresults in low stratocumulus clouds and occasionalsnow flurries (see fig. 4-6).

A particularly troublesome situation often ariseswhen the cold air flows from a cold, snow-coveredsurface to a water surface and then over a cold,snow-covered surface again. This frequently happenswith air crossing the Great Lakes. (See fig. 4-7.)

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150 O

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Figure 4-5.—Trajectories of cP and cA air in winter.

CP

20 TO 30 MPHTURBULENCE

SNOWFLURRIES

CLEAR AND COLD

N S

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Figure 4-6.—cP air moving southward.

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TEMP.DIFFERENCE + 10 F

O

TEMP. 2000' 10 FO

MOISTURE ANDWARM

AIR RISING

O

SURFACE TEMP. 0 F

NW SE

WATER TEMP. 34 FO

SURFACE TEMP. 21 FO

CLEAR

SNOW FLURRIESAPPALACHIANMOUNTAINS

10 FO

3C/1000'

O

AG5f0407

Figure 4-7.—cP air moving over the Great Lakes (winter).

On the leeward side of the Great Lakes and on thewindward side of the Appalachians, you can expect arather low, broken to overcast sky condition withfrequent and widespread snow squalls. Stratocumulusand cumulus clouds with bases at 500 to 1,000 feet andtops at 7,000 to 10,000 feet form on the leeward side ofthe Great Lakes. Over the mountains, their tops extendto about 14,000 feet. Visibility ranges from 1 to 5 milesduring rain or snow showers and occasionally lowers tozero in snow flurries.

Severe aircraft icing conditions may be expectedover the mountains and light to moderate aircraft icingon the leeward side of the lakes. Moderate to severeflying conditions are the rule as long as the outflow ofcold air continues.

East of the Appalachians, skies are relatively clearexcept for scattered stratocumulus clouds. Visibility isunrestricted and the surface temperature is relativelymoderate because of turbulent mixing. In the MiddleWest, clouds associated with this type of air masscontinue for 24 to 48 hours after the arrival of the coldmass, while along the Atlantic Coast rapid passage ofthe leading edge of the air mass produces almostimmediate clearing.

TRAJECTORY PATHS C AND D (ANTI-CYCLONIC).—The weather conditions experiencedover the central United States under the influence oftrajectories similar to C and D (fig. 4-5) are quitedifferent. Unusually smooth flying conditions arefound in this region, except near the surface where aturbulence layer results in a steep lapse rate and somebumpiness. Low stratus or stratocumulus clouds in mayform at the top of the turbulence layer. As the cold airstagnates and subsides under the influence of theanticyclonic trajectory, marked the haze layers developindicating the presence of subsidence inversions. Thesurface visibility also deteriorates because of anaccumulation of smoke and dust as the air stagnates andsubsides. This is especially noticeable during the earlymorning hours when the stability in the surface layers ismost pronounced. In the afternoon, when surfaceheating has reached a maximum, the visibility usuallyimproves because of the steep lapse rate and resultantturbulence.

Movement of cPk and cAk air westward over theRocky Mountains to the Pacific coast is infrequent.However, when successive outbreaks of cold air buildup a deep layer of cP air on the eastern slopes of theRocky Mountains, relatively cold air can flow towardthe Pacific coast.

TRAJECTORY PATH E.—When the trajectoryof the cold air is similar to E in figure 4-5, rather mildtemperatures and low humidities result on the Pacificcoast because adiabatic warming of the air flowingdown the mountain slopes produces clear skies andgood visibility.

TRAJECTORY PATHS F AND G.—Occasionally, the trajectory passes out over the PacificOcean (see fig. 4-5). The air then arrives over centraland southern California as cold, convectively unstableair. This type is characterized by squalls and showers,cumulus and cumulonimbus clouds, visibility of 1 to 5miles during squalls and showers, and snow even as farsouth as southern California.

Maritime Polar (mP) Air Pacific in Winter

Maritime polar air from the Pacific dominates theweather conditions of the west coast during the wintermonths. In fact, this air often influences the weatherover most of the United States. Pacific coastal weather,while under the influence of the same general air mass,varies considerably as a result of different trajectoriesof mP air over the Pacific. Thus knowledge oftrajectories is of paramount importance in forecastingwest coast weather.

When an outbreak of polar air moves over only asmall part of the Pacific Ocean before reaching theUnited States, it usually resembles maritime arctic cold(mAk). If its path has been far to the south, it is typicallymP. Figure 4-8 shows some of the trajectories (A, B, C,D) by which mP air reaches the North American coastduring the winter.

TRAJECTORY PATH A (CYCLONIC).—Trajectory path A air originates in Alaska or northernCanada and is pulled out over the Pacific Ocean by alow center close to British Columbia in the Gulf ofAlaska. This air has a relatively short overwater pathand brings very cold weather to the Pacific Northwest.When the air reaches the coast of British Columbia andWashington after 2 to 3 days over the water, it isconvectively unstable. This instability is released whenthe air is lifted by the coastal mountain ranges. Showersand squalls are common with this condition. Ceilingsare generally on the order of 1,000 to 3,000 feet alongthe coast and generally 0 over the coastal mountainranges. Cumulus and cumulonimbus are thepredominating cloud types, and they generally extendto very high levels. Visibility is generally good becauseof turbulence and high winds commonly found withthis trajectory. Of course, in areas of precipitation, the

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visibility is low. Icing conditions, generally quitesevere, are present in the clouds. After this mP air hasbeen over land for several days, it has stabilized andweather conditions improve significantly.

TRAJECTORY PATHS B AND C(CYCLONIC).—Trajectory paths B and C air with alonger overwater trajectory dominate the west coast ofthe United States during winter months. When there israpid west-to-east motion and small north-to-southmotion of pressure systems, mP air may influence theweather over most of the United States. Because of alonger overwater trajectory, this mP air is heated togreater heights, and convective instability is present upto about 10,000 feet.

This air has typical k characteristics—turbulentgusty winds, steep lapse rate, good visibility at groundexcept 0 to 3 miles in precipitation, as well as cumulusand cumulonimbus clouds with showers. Theseshowers are not as intense as those produced in theshorter trajectory mP air, but the total amount ofprecipitation is greater.

TRAJECTORY PATH D (ANTI-CYCLONIC).—This trajectory usually is over waterlong enough to permit modifications to reachequilibrium at all levels. When the air reaches the coast,it is very stable with one or two subsidence inversions.Stratus or stratocumulus clouds are frequently found.Ceilings are usually 500 to 1,500 feet and the tops ofclouds are generally less than 4,000 feet. Visibility isfair except during the early morning hours when hazeand smoke reduce the visibility to less than 1 mile. Thistype of air is found over the entire Pacific coast. It isincorrectly referred to as mT air, since it follows thenorthern boundary of the Pacific anticyclone. However,mT air does on rare occasions move into Californiaalong this path.

Gradually mP air drifts eastward with theprevailing west-east circulation. In crossing the coastalranges and the Rocky Mountains, much of the moisturein the lower layers is condensed out; the heat ofcondensation liberated is absorbed by the intermediatelayers of air. On the eastern slopes of the mountains, theair is warmed as it descends dry-adiabatically. As itflows over the cold and often snow-covered landsurface east of the mountains, the warm mP airbecomes stable in the lower layers.

The flying conditions in mP air east of the RockyMountains are in general the best that are experiencedin winter. Relatively large diurnal temperature rangesare observed. Turbulence is almost absent and visibilityis good, except for the smoke and haze in industrialareas. Ceilings are generally unlimited, since either noclouds or only a few high clouds are present. This typeof mild winter weather occasionally spreads eastwardto the Atlantic coast. When mP air crosses the RockyMountains and encounters a deep, dense dome of cP air,it is forced to overrun it and results in storm conditionsthat produce blizzards over the plains states.

Maritime Tropical (mT) Air Pacific in Winter

Maritime tropical (mT) air is observed onlyinfrequently on the Pacific coast, particularly near thesurface. Air flowing around the northern boundary ofthe Pacific anticyclone is at times mT air but is usuallymP air. This air has the weather characteristics (as wellas the low temperature) of mP air, having had a longtrajectory over the water. (See fig. 4-9.)

Occasionally the eastern cell of the Pacificanticyclone splits, and one portion moves southwardoff the coast of southern California. This portion of theanticyclone is then able to produce an influx of mT air.

4-10

50

O

AG5f0408

50

O

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O

180

O

17

0O

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O 17

0O

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O160

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60

O

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150 O

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60 O

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120 O

120 O

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100O

40 O

30

O30

O

30

O

30O

30 O

30 O

130 O

30 O

20 O

AB

C

D

17

0O

Figure 4-8.—Trajectories of mP air over the Pacific Coast inwinter.

Generally the influx of mT air is carried aloft by arapidly occluding frontal system somewhere oversouthern California, producing the heaviestprecipitation recorded in that area. Occasionally mT airis seen above the surface with pronounced stormdevelopments over the Great Basin. Since large, open,warm sectors of mT air do not occur along the westcoast, representative air mass weather is notexperienced. Flying conditions are generally restrictedwhen this air is present, mainly because of low frontalclouds and reduced visibility in precipitation areas.

Maritime Polar (mP) Air Atlantic in Winter

Maritime polar air, which originates in the Atlantic,becomes significant at times along the east coast. It isnot nearly so frequent over North America as the othertypes because of the normal west-east movement of allair masses. This type of air is observed over the eastcoast in the lower layers of the atmosphere whenever acP anticyclone moves slowly off the coast of themaritime provinces and New England. (See fig. 4-10.)This air, originally cP, undergoes less heating than itsPacific counterpart because the water temperatures arecolder and also because it spends less time over thewater. This results in the instability being confined tothe lower layers of this air. The intermediate layers ofthis air are very stable. Showers are generally absent;however, light drizzle or snow and low visibility arecommon. Ceilings are generally about 700 to 1,500 feet

with tops of the clouds near 3,000 feet. Markedsubsidence above the inversion ensures that cloudscaused by convection will not exist above that level.

The synoptic weather condition favorable to mP airover the east coast is usually also ideal for the rapiddevelopment of a warm front with maritime tropical airto the south. Maritime tropical air then overruns the mPair and a thick cloud deck forms. Clouds extendingfrom near the surface to at least 15,000 feet areobserved. Ceilings are near zero and severe icingconditions exist in the cold air mass. Frequently,freezing rain and sleet are observed on the ground.Towering cumulus clouds prevail in the warm air andoften produce thunderstorms.

Flying conditions are rather dangerous with mP airbecause of turbulence and icing conditions present nearthe surface. Poor visibility and low ceilings areadditional hazards. The cloudiness associated with themP air mass usually extends as far west as theAppalachians.

Maritime Tropical (mT) Air Atlantic in Winter

Temperature and moisture content are higher in mTair masses than in any other American air mass inwinter. In the southern states, along the Atlantic coast

4-11

AG5f0409

140 O

50 O

mT

150 O

130O

120O

120O130

O

40 O

40 O

40 O

40O

110O

130O

140 O

30 O

30 O

30O

30O

120O

140 O

50 O

mP

Figure 4-9.—Trajectory of mT air over the Pacific in winter.AG5f0410

40O

cP50

O

50O

50O

50O

70O

50O 50

O

60O

80O

60O

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60O

60O

70O

80O

90O

30O 30

O

30O

mP

90O 70O

NOTE: The representation of internationalboundaries on this chart is not necessarilyauthoritative.

Figure 4-10.—Trajectory of mP air over the Atlantic in winter.

and Gulf of Mexico (fig. 4-11), mild temperatures, highhumidities, and cloudiness are found, especially duringthe night and early morning. This is the characteristicweather found in mT air in the absence of frontalconditions. The stratus and stratocumulus clouds thatform at night tend to dissipate during the middle of theday and fair weather prevails. Visibility is generallypoor when the cloudiness is present; however, itimproves rapidly because of convective activity whenthe stratus clouds dissipate. The ceilings associatedwith the stratus condition generally range from 500 to1,500 feet, and the tops are usually not higher than3,500 to 4,500 feet. Precipitation does not occur in the

absence of frontal action. With frontal activity, theconvective instability inherent in this air is released,producing copious precipitation.

If mT air is forced over mountainous terrain, as inthe eastern part of the United States, the conditionalinstability of the air is released at higher levels. Thismight produce thunderstorms or at least largecumuliform clouds. (See fig. 4-12.) Pilots must beaware that these clouds may develop out of stratiformcloud systems and therefore may occur withoutwarning. Icing may also be present. Thus, in the GreatLakes area, a combination of all three hazards (fog,thunderstorms, and icing) is possible.

Occasionally when land has been cooled along thecoastal area in winter, maritime tropical air flowinginland produces an advection fog over extensive areas.(See fig. 4-13.) In general, flying conditions under thissituation are fair. Ceilings and visibilities areoccasionally below safe operating limits; however,flying conditions are relatively smooth and icingconditions are absent near the surface layers.

As the trajectory carries the mT air northward overprogressively colder ground, the surface layers cool andbecome saturated. This cooling is greatly accelerated ifthe surface is snow or ice covered or if the trajectorycarries the air over a cold-water surface. Depending onthe strength of the air mass, fog with light winds or alow stratus deck with moderate to strong winds formsrapidly because of surface cooling. Occasionallydrizzle falls from this cloud form; and visibility, evenwith moderate winds, is poor. Frontal lifting of mT airin winter, even after the surface layers have becomestabilized, results in copious precipitation in the form ofrain or snow.

4-12

AG5f0411

mT

40O

100O

40O

90O

80O

50O 50

O

70O

40O 40

O

70O

30O

30O

80O

30O

100O

80O

70O

20O

mT20

O

Figure 4-11.—Trajectories of mT air over the Atlantic inwinter.

WATER TEMP. 20 C.O

GULF OF MEXICO DRIZZLE AND FOG

SW NE

APPALACHIANS

AG5f0412

10 C. SURFACE TEMP. 0 C.O O

STRATUSmT

CUMULONIMBUS

Figure 4-12.—mT air moving northeastward.

During the winter, air resembling mT isoccasionally observed flowing inland over the gulf andsouth Atlantic states. Generally the air that had arelatively short trajectory over the warm waters off thesoutheast coast is cP air. Clear weather usuallyaccompanies cP air in contrast to cloudy weatheraccompanying a deep current of mT air. On surfacesynoptic charts, the apparent mT air can bedistinguished from true mT air by the surface dew-pointtemperature value. True mT air always has dew-pointtemperature values in excess of 60°F. The highlymodified cP air usually has dew-point values between50°F and 60°F.

NORTH AMERICAN AIR MASSES,TRAJECTORIES, AND WEATHER(SUMMER)

During the summer most of the United States isdominated by either S or mT air, whereas Canada andthe northwestern United States are dominated by polarair. Occasionally, tropical air is transported to theCanadian tundra and Hudson Bay region.

Continental Polar (cP) Air in Summer

Continental polar (cP) air has characteristics andproperties quite different from those of its wintercounterpart. Because of the long days and the higheraltitude of the sun (as well as the absence of a snowcover over the source region), this air is usuallyunstable in the surface layers, in contrast to the markedstability found in cP air at its source in winter. By thetime this air reaches the United States, it can no longerbe distinguished from air coming in from the NorthPacific or from the Arctic Ocean. (See fig. 4-14.)

Clear skies or scattered cumulus clouds withunlimited ceilings characterize this mass at its sourceregion. Occasionally, when this air arrives over thecentral and eastern portion of the United States, it is

4-13

WARM OCEAN COLD CONTINENT

FOG OR LOW STRATUS

AG5f0413

Figure 4-13.—mT (Gulf of Mexico or Atlantic) air of wintermoving northward over cold continent.

AG5f0414

cP

70O

60O60

O

80O

60O

50O

50O

100O

50O

120 O

120 O

130 O

130 O

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50 O

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40 O

40O

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120 O

120 O

30 O

30O

30O

110O

110O

100O

100O

40O

40O90

O

80O

80O

90O

30O

70O

40O

50 O

50 O

Figure 4-14.—Continental polar (cP) air in summer.

characterized by early-morning ground fogs or lowstratus decks. Visibility is generally good except whenhaze or ground fog occurs near sunrise. Convectiveactivity, usually observed during the daytime, ensuresthat no great amounts of smoke or dust accumulate inthe surface layers. An exception to this is found understagnant conditions near industrial areas, whererestricted visibility may occur during the day and night.Pronounced surface diurnal temperature variations areobserved in cP air during summer.

The convective activity of this air is generallyconfined to the lower 7,000 to 10,000 feet. Flyingconditions are generally smooth above approximately10,000 feet except when local showers develop.Showers, when observed, usually develop in a modifiedtype of cPk over the southeastern part of the country.The base of cumulus clouds that form in this air isusually about 4,000 feet because of the relative drynessof this air mass.

Maritime Polar (mP) Air Pacific in Summer

The entire Pacific coast is usually under theinfluence of mP air in the summer. (See fig. 4-15.) Witha fresh inflow of mP air over the Pacific coast, clearskies or a few scattered cumulus are generally observedover the coastal mountains. As this air flows southwardalong the coast, a marked turbulence inversionreinforced by subsidence from aloft is observed. Stratusor stratocumulus clouds generally form at the base ofthe inversion. Ceilings are generally 500 to 1,500 feetwith tops of clouds seldom above 3,500 feet. The

formation of the stratus condition along the coast ofCalifornia is greatly enhanced by the presence of theupwelling of cold water along the coast. East of theRocky Mountains, this air has the same properties as cPair.

Maritime Polar (mP) Air Atlantic in Summer

In spring and summer, mP air is occasionallyobserved over the east coast. Marked drops intemperature that frequently bring relief from heatwaves usually accompany the influx of this air (fig.4-16). Just as in winter, there is a steep lapse rate in thelower 3,000 feet of this mass. Stratiform clouds usuallymark the inversion. Ceilings are from 500 to 1,500 feet,and the tops of the clouds are usually 1,000 to 2,500feet. No precipitation occurs from these cloud types andvisibility is usually good. This air usually does notconstitute a severe hazard to flying.

Maritime Tropical (mT) Air Pacific in Summer

Maritime tropical (mT) Pacific air has no directinfluence on the weather over the Pacific coast. Duringthe summer season, the Pacific anticyclone movesnorthward and dominates the Pacific Coast weatherwith mP air. Occasionally mT air reaches the WestCoast; for example, tropical storms or typhoonssometimes move northerly along the Baja Coast. Thissynoptic condition produces a great amount ofcloudiness and precipitation.

4-14

AG5f0415

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O

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Figure 4-15.—Trajectories of mP air over the Pacific insummer.

AG5f0416

mP

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Figure 4-16.—Trajectories of mP air over the Atlantic insummer.

Maritime Tropical (mT) Air Atlantic in Summer

The weather in the eastern half of the United Statesis dominated by mT air in summer (fig. 4-17). As inwinter, warmth and high moisture content characterizethis air. In summer, convective instability extends tohigher levels; there is also a tendency toward increasinginstability when the air moves over a warmer landmass.(See fig. 4-18.) This is contrary to winter conditions.

Along the coastal area of the southern states, thedevelopment of stratocumulus clouds during the earlymorning is typical. These clouds tend to dissipateduring the middle of the morning and immediatelyreform in the shape of scattered cumulus. Thecontinued development of these clouds leads toscattered showers and thunderstorms during the lateafternoon. Ceilings in the stratocumulus clouds aregenerally favorable (700 to 1,500 feet) for the operationof aircraft. Ceilings become unlimited with thedevelopment of the cumulus clouds. Flying conditionsare generally favorable despite the shower andthunderstorm conditions, since the convective activityis scattered and can be circumnavigated. Visibility isusually good except near sunrise when the air isrelatively stable over land.

When mT air moves slowly northward over thecontinent, ground fogs frequently form at night. Seafogs develop whenever this air flows over a relativelycold current such as that occurring off the east coast.The notorious fogs over the Grand Banks ofNewfoundland are usually formed by this process.

In late summer, the Bermuda high intensifies attimes and seems to retrograde westward. This results ina general flow of mT air over Texas, New Mexico,Arizona, Utah, Colorado, and even southern California.The mT air reaching these areas is very unstablebecause of the intense surface heating and orographiclifting it undergoes after leaving the source region in theCaribbean and Gulf of Mexico. Shower andthunderstorm conditions, frequently of cloudburstintensity, then prevail over the southwestern states.Locally this condition is termed sonora weather.

4-15

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Figure 4-17.—Maritime tropical (mT) air, Atlantic, in summer.

WARM OCEAN WARMER CONTINENT

AG5f0418

Figure 4-18.—mT (Gulf of Mexico or Atlantic) air in summermoving northward over a warm continental surface.

Continental Tropical (cT) Air in Summer

Continental tropical air is found over the UnitedStates only in the summer. Its source region is therelatively small area over the northern portion ofMexico, western Texas, New Mexico, and easternArizona. High surface temperatures and very lowhumidities are the main air mass characteristics. Largediurnal temperature ranges and the absence ofprecipitation are additional properties of cT air. Flyingconditions are excellent. However, during the daytimeturbulence sometimes extends from the surfacethroughout the average flying levels.

Superior (S) Air in Summer

Superior air usually exists over the southwesternstates and is believed to be the result of strong subsidingmotions. Most frequently this air is observed above aninversion layer at high levels; it rarely descends to thesurface. Above the inversion layer, this superior air isthe warmest air mass observed in the United States at itsaltitude; but, because of its steep lapse rate, itstemperature at higher levels is less than that of tropicalair. Relative humidity is usually less than 30 percent.Quite often they are too low to measure accurately.

Superior air is observed in both summer and winter.Flying conditions are excellent in this air mass, since nocloud forms are present and visibilities are usually verygood because of the dryness. This type of air mass isvery important because superior air frequently stops allconvective activity caused by intruding maritimetropical air. This generally prevents the formation ofshowers and thunderstorms unless the mT air mass isdeep.

NOTE: Views A and B of figure 4-19 show theproperties of significant North American air massesduring the winter and summer seasons from thestandpoint of flying.

AIR MASSES OVER ASIA

The air masses commonly observed over Asia(especially eastern Asia) are continental polar,maritime tropical, and equatorial. Maritime polar andcontinental tropical air play a minor part in the air masscycle of Asia.

Continental Polar (cP) Air

Continental polar air, as observed over the interiorof Asia, is the coldest air on record in the Northern

Hemisphere. This is brought about by the fact that theinterior of Asia, made up of vast level and treelessregions, serves as an ideal source region. The Himalayamountain range, across southern Asia, aids in theproduction of cP air. It tends to keep the polar air overthe source region for a long time and to block the inflowof tropical air from the lower latitudes.

The weather conditions over eastern Asia aregoverned by this air mass throughout the winter.Successive outbreaks of this air occur over Siberia,China, and the Japanese Islands and establish the winterweather pattern. The weather conditions prevailing inthis air are similar to those found in cP air over theeastern portion of North America.

The cold air that is forced southward over theHimalaya Mountains to India and Burma arrives in ahighly modified form and is known as the wintermonsoon. The weather conditions during the wintermonsoon are dominated by the dry and adiabaticallywarmed polar air flowing equator-ward. It is whileunder the influence of these monsoon conditions thatgenerally pleasant weather prevails over most of thearea.

Maritime Tropical (mT) Air

Maritime tropical air is usually observed along thecoast of China and over the Japanese Islands during thesummer. In structure it is almost identical to the mT airobserved off the east coast of North America. Theweather conditions found in this air are similar to thoseof its North American counterpart.

Equatorial (E) Air

Equatorial air is observed over southeastern Asia.During the summer all of India and Burma are under theinfluence of E air, because of the summer monsooncirculation. In the wintertime, when offshore windsprevail, E air is not found over the landmasses but isfound some distance offshore. Equatorial air is anextremely warm and moist air mass. It has great verticaldepth, often extending beyond 20,000 feet in height.This entire column is unstable, and any slight lifting orsmall amount of surface heating tends to release theinstability and produce showers and squalls. Theequatorial air observed over India and Burma is almostidentical in structure with E air found all along theequatorial zone over the entire Earth. Unmodifiedequatorial air is observed over India and Burma duringthe summer monsoon.

4-16

4-17

cP (near sourceregion)

cP (southeast ofGreat Lakes)

mP (on Pacificcoast)

mP (east ofRockies)

mP (east coast)

mT (Pacificcoast)

mT (east ofRockies)

None

Stratocumulus and cumulustops 7,000-10,000 feet.

Cumulus tops above20,000 feet.

None

Stratocumulus and stratustops 6,000-8,000 feet.

Stratus or stratocumulus.

Stratus or stratocumulus

Unlimited

500-1,000 feet,0 over mountains.

1,000-3,000 feet,0 over mountains.

Unlimited

0-1,000 feet

500-1,500 feet

100-1,5000 feet

Excellent (except near indus-trial areas, then 1-4 miles).

1-5 miles, 0 in snowflurries.

Good except 0 over mountainsand in showers.

Excellent except near indus-trial areas, then 1-4 miles.

Fair except 0 in precipi-tation area.

Good

Good

Smooth except withhigh winds velocities.

Moderate turbulenceup to 10,000 feet.

Moderate to strongturbulence.

Smooth except in lowerlevels with high winds.

Rough in lower levels.

Smooth

Smooth

-10 to -60.

0 to 20.

45 to 55.

30 to 40.

30 to 40.

55 to 60.

60 to 70.

WINTERAIR MASSES CLOUDS CEILINGS VISIBILITIES TURBULENCE

SURFACETEMPERA-TURE F.A

cP (near sourceregion)

cP (Pacificcoast)

mP (east ofPacific)

S (MississippiValley)

mT (east ofRockies)

Scattered cumulus.

Stratus tops,2,000-5,000 feet.

None except scattered cumu-lus near mountains.

None

Stratocumulus early morning;cumulonimbus afternoon.

Unlimited

100 feet-2,500 feet, un-limited during day.

Unlimited

Unlimited

500-1,000 feet a.m.;3,000-4,000 feet p.m.

Good

1/2 - 10 miles

Excellent

Excellent

Excellent

Moderate turbulence up to10,000 feet.

Slightly rough in clouds.Smooth above.

Generally smooth except overdesert regions in afternoon.

Slightly rough up to 15,000feet.

Smooth except in thunder-strorms, then strong tur-bulence.

55-60

50-60

60-70

75-85

75-85

SUMMERAIR MASSES CLOUDS CEILINGS VISIBILITIES TURBULENCE

SURFACETEMPERA-TURE F.B

AG5f0419

Figure 4-19.—Properties of significant air masses over North America from the standpoint of flying—(A) Winter; (B) Summer.

The weather conditions during the summermonsoon consist of cloudy weather with almostcontinuous rain and widespread shower activity. Hightemperatures and high humidities further add to thediscomfort.

AIR MASSES OVER EUROPE

Although, in general, the characteristics of airmasses over Europe are much the same as those foundover North America, certain differences do exist. Onereason for this is that an open ocean extends betweenEurope and North America toward the Arctic. Thisallows an influx of mA air to reach Europe. This type ofair is not encountered over North America. Thelocation of an extensive mountain range in an east-westdirection across southern Europe is an additionalinfluence not present over North America, where theprevailing ranges are oriented in a north-southdirection.

If the trajectory of the air is observed carefully andthe modifying influences of the underlying surface areknown, it is easy to understand the weather and flyingconditions that occur in an air mass over any continentor ocean.

Maritime Arctic (mA) Air in Winter

Maritime arctic air is observed primarily overwestern Europe. Strong outbreaks of this air,originating in the Arctic between Greenland andSpitsbergen, usually follow a cyclonic trajectory intowestern Europe.

Because of their moisture content and instability,cumulus and cumulonimbus clouds are typical of thisair mass, frequently producing widespread showers andsqualls. Visibility is generally good, but moderate tosevere icing often affects aircraft operations.

With the presence of a secondary cyclonic systemover France or Belgium, mA air occasionally sweepssouthward across France to the Mediterranean, givingrise to the severe mistral winds of the Rhone Valley andthe Gulf of Lyons. Heavy shower and thunderstormconditions are typical in this situation.

Maritime Arctic (mA) Air in Summer

In summer, this air is so shallow that in movingsouthward from its source region, it modifies to thepoint where it can no longer be identified and is thenindicated as mP air.

Maritime Polar (mP) Air in Winter

Maritime polar air observed over Europe usuallyoriginates in the form of cP air over North America. Itreaches the west coast of Europe by various trajectoriesand is found in different stages of modification; itproduces weather similar to mP air over the west coastof North America.

Maritime Polar (mP) Air in Summer

Maritime polar air observed over Europe is similarto mP air observed on the west coast of North America.The weather conditions associated with this air aregenerally good. Occasionally, because of surfaceheating, a shower or thunderstorm is observed in thedaytime over land.

Continental Arctic (cA) and Continental Polar(cP) Air in Winter

The source region for cA and cP air is over northernRussia, Finland, and Lapland. The cA and cP airmasses are observed over Europe in connection with ananticyclone centered over northern Russia and Finland.Occasionally they reach the British Isles and at timesextend southward to the Mediterranean.

Because of the dryness of cA and cP air, clouds areusually absent over the continent. Fair-weathercumulus are the typical clouds when cA and cP air areobserved over the British Isles. Over theMediterranean, cA and cP air soon become unstableand give rise to cumulus and cumulonimbus cloudswith showers. Occasionally these air masses initiate thedevelopment of deep cyclonic systems over the centralMediterranean. Visibility is usually good; however,after this type becomes modified, haze layers form andreduce the visibility.

Continental Arctic (cA) and Continental Polar(cP) Air in Summer

The source region for cA and cP air is the same asfor its counterpart in winter. It is a predominantly dryair mass and produces generally fair weather over thecontinent and the British Isles. The visibility is usuallyreduced because of haze and smoke in the surfacelayers. As cA and cP air stream southward, the lowerlayers become unstable; and eventually convectiveclouds and showers develop in the later stages of theirlife cycles.

4-18

Maritime Tropical (mT) Air in Winter

Maritime tropical air that arrives over Europeusually originates over the southern portion of theNorth Atlantic under the influence of the Azoresanticyclone. Maritime tropical air is marked bypronounced stability in the lower layers and typicalwarm-mass cloud and weather conditions. Relativelyhigh temperatures accompany the influx of mT air, andthe moisture content is greater than in any other airmass observed in the middle latitudes of Europe.Visibility is, as a rule, reduced because of the presenceof fog and drizzle, which are frequently observed withan influx of mT air. Maritime tropical air in winterexists only in western Europe. By the time it reachesRussia, it is generally found aloft and greatly modified.

Maritime Tropical (mT) Air in Summer

In general, mT air has the same properties as itscounterpart in winter with the exception that it is lessstable over land because of surface heating.Additionally, this air mass loses its maritimecharacteristics soon after passing inland.

Over water, mT air is still a typical warm air mass.Sea fog frequently occurs in the approaches to theEnglish Channel during the spring and early summer.Visibility in mT air is generally better in summer thanin winter, particularly over land where convectioncurrents usually develop.

Maritime tropical air flowing over theMediterranean in summer usually changes to a coldmass, since the water temperature of the Mediterraneanis then slightly higher than that of the air. Weakconvection currents prevail, usually sufficiently strongto form cumulus clouds but seldom sufficiently strongto produce showers.

Continental Tropical (cT) Air in Winter

The continental tropical air that arrives over Europein winter originates over North Africa. By the time itreaches central Europe, it differs little from mT air. Ingeneral, a cT air mass is much more prevalent oversouthern Europe than over central or western Europe.Although the moisture content of cT air is less than thatobserved in mT air, the visibility is not much better,primarily because of the dust that cT air picks up whileover Africa. This air mass constitutes the major sourceof heat for the development of the Mediterraneancyclonic storms, most common during the winter andspring months.

Continental Tropical (cT) Air in Summer

The cT air usually develops over North Africa, AsiaMinor, and the southern Balkans. At its source region,the air is dry and warm as well as unstable. The NorthAfrican air mass is the hottest air mass on record in theworld. In its northward flow over southern Europe, cTair absorbs moisture and increases its convectiveinstability. The summer showers and thunderstormsobserved over southern Europe are often produced by amodified cT air mass. This air mass is much moreprevalent over southern Europe than is its wintercounterpart.

AIR MASSES IN THE SOUTHERNHEMISPHERE

The air masses of the Southern Hemisphere arepredominantly maritime. This is because of theoverwhelming preponderance of ocean areas. Greatmeridional (south-north and north-south) transports ofair masses, as they are known in the NorthernHemisphere, are absent because the westerlies aremuch more developed in the Southern Hemisphere thanin the Northern Hemisphere. Except for Antarctica,there are no large landmasses in the high latitudes in theSouthern Hemisphere; this prevents sizable invasionsof antarctic air masses. The large landmasses near theequator, on the other hand, permit the extensivedevelopment of warm air masses.

The maritime tropical air masses of the SouthernHemisphere are quite similar to their counterparts ofthe Northern Hemisphere. In the large area of Brazil,there are two air masses for consideration. One is theregular air mass from the Atlantic, which is composedof unmodified mT air. The other originates in theAtlantic; but by the time it spreads over the hugeAmazon River basin, it undergoes two importantchanges—the addition of heat and moisture. As a resultof strong summer heating, a warm, dry continentaltropical (cT) air mass is located from 30° south to 40°south.

The maritime polar air that invades South Americais quite similar to its counterpart in the United States.Maritime polar air occupies by far the most territory inthe Southern Hemisphere, encircling it entirely.

Australia is a source region for continental tropicalair. It originates over the vast desert area in the interior.Except along the eastern coast, maritime tropical airdoes not invade Australia to a marked degree. This air isbrought down from the north, particularly in the

4-19

summer, by the counterclockwise circulation aroundthe South Pacific high.

Antarctica is a great source region for intenselycold air masses. The temperatures are colder than in thearctic regions. These air masses have continentalcharacteristics, but before the air reaches other landareas, it becomes modified and is properly calledmaritime polar.

During the polar night the absence of insolationcauses a prolonged cooling of the snow surface, whichmakes Antarctica a permanent source of very cold air. Itis extremely dry and stable aloft. This polar air mass isreferred to as continental antarctic (cA) air. In summerthe continent is not as cold as in winter because ofconstant solar radiation but continues to function as asource for cold cA air.

In both winter and summer, the air mass isthermally modified as it flows northward throughdownslope motion and surface heating; as a result, itbecomes less stable. It assumes the characteristics ofmaritime antarctic air. The leading edge of this air massthen becomes the northern boundary of the antarcticfront.

To the north of the antarctic front is found a vastmass of maritime polar air that extends around thehemisphere between 40°S and 68°S in summer andbetween 34°S and 65°S in winter. At the northern limitof this air mass is found the Southern Hemisphere polarfront. During summer this mP air is by far the mostimportant cold air mass of the hemisphere because ofthe lack of massive outbreaks of cold continental airfrom Antarctica.

Different weather conditions occur with each typeof air mass. The cA air produces mostly clear skies. ThemA air masses are characterized generally by anextensive overcast of stratus and stratocumulus cloudswith copious snow showers within the broad zone of theantarctic front. An area of transition that extendsmainly from the coastline to the northern edge of theconsolidated pack ice is characterized by broken toovercast stratocumulus clouds with somewhat higherbases and little precipitation.

REVIEW QUESTIONS

Q4-1. What is the definition of an air mass?

Q4-2. Name the two factors that are necessary toproduce an air mass?

Q4-3. What type of air mass is mTk?

Q4-4. What are the two modifying influences on airmasses?

Q4-5. What is the warmest air mass observed in theUnited States at its altitude?

FRONTS

LEARNING OBJECTIVE: Describe thespecific parts that make up a front and identifyhow a front is classified as either cold, warm,occluded, or quasi-stationary.

A front, generally speaking, is a zone of transitionbetween two air masses of different density andtemperature and is associated with major weatherchanges, some of which can be violent. This fact aloneis sufficient reason for an in-depth study of fronts andtheir relationship to air masses and cyclones.

DEFINITIONS AND CLASSIFICATIONS

A front is not just a colorful line drawn on a surfacechart. A front is a three-dimensional phenomena with avery specific composition. Since a front is a zone oftransition between two air masses of different densities,there must be some sort of boundary between these airmasses. One of these boundaries is the frontal surface.The frontal surface is the surface that separates the twoair masses. It is the surface next to the warmer air (lessdense air). In reality, however, the point at which two airmasses touch is not a nice, abrupt separation. This areais a zone of a large density gradient. This zone is calledthe frontal zone. A frontal zone is the transition zonebetween two adjacent air masses of different densities,bounded by a frontal surface. Since the temperaturedistribution is the most important regulator ofatmospheric density, a front almost invariably separatesair masses of different temperatures.

At this point you should be aware of the varioustypes of fronts. The question in your mind should behow a front is classified. Whether it is cold, warm, orstationary. A front is classified by determining theinstantaneous movement. The direction of movementof the front for the past 3 to 6 hours is often used.Classification is based on movement relative to thewarm and cold air masses involved. The criterion is asfollows:

Cold Front

A cold front is one that moves in a direction inwhich cold air displaces warm air at the surface. Inother words the cold (or cooler) air mass is moving

4-20

toward a warmer air mass. The cooler, denser air issliding under the warmer, less dense air displacing itupward.

Warm Front

A warm front is one along which warmer airreplaces colder air. In this case, a warmer air mass ismoving toward a cooler retreating air mass. Thewarmer, less dense air moves only toward and replacesthe colder, denser air if the colder air mass is alsomoving.

Quasi-Stationary Front

This type front is one along which one air massdoes not appreciably replace the other. These fronts arestationary or nearly so (speed under 5 knots). They canmove or undulate toward either the cold or warm airmass.

Occluded Front

An occluded front is one where a cold frontovertakes a warm front, forcing the warm air upward.The occluded front may be either a warm front or a coldfront type. a warm front type is one in which the cool airbehind the cold front overrides the colder air in advanceof the warm front, resulting in a cold front aloft. A coldfront type is one in which the cold air behind the coldfront under rides the warm front, resulting in a warmfront aloft.

RELATION OF FRONTS TO AIRMASSES AND CYCLONES

LEARNING OBJECTIVE: Describe therelationship of fronts to air masses and stableand unstable wave cyclones.

RELATION OF FRONTS TO AIR MASSES

At this point you should have figured out thatwithout air masses there would be no fronts. Thecenters of action are responsible for bringing the airmasses together and forming frontal zones.

The primary frontal zones of the NorthernHemisphere are the arctic frontal zone and the polarfrontal zone. The most important frontal zone affectingthe United States is the polar front. The polar front isthe region of transition between the cold polar air andwarm tropical air. During the winter months (in theNorthern Hemisphere), the polar front pushes farthersouthward, because of the greater density of the polarair, than during the summer months. During thesummer months (in the Northern Hemisphere), thepolar front seldom moves farther south than the centralUnited States.

On a surface map a front is indicated as a lineseparating two air masses; this is only a picture of thesurface conditions. These air masses and fronts extendvertically. (See fig. 4-20.)

A cold air mass, being heavier, acts like a wedgeand tends to under run a warm air mass. Thus, the coldair is below and the warm air is above the surface ofdiscontinuity. This wedge of cold air produces a slopeof the frontal surface. This slope is usually between 1 to50 (1-mile vertical for 50 miles horizontal) for a coldfront and 1 to 300 (1-mile vertical for 300 mileshorizontal) for a warm front. For example, 100 milesfrom the place where the frontal surface meets theground, the frontal surface might be somewherebetween 2,000 feet and 10,000 feet above Earth’ssurface, depending on the slope. The slope of a front isof considerable importance in visualizing andunderstanding the weather along the front.

4-21

COLD-FRONTSLOPE

COLD AIR

WARM AIR

WARM-FRONTSLOPE

COLD AIR

AG5f0420

Figure 4-20.—Vertical view of a frontal system (clouds not shown).

RELATION OF FRONTS TO CYCLONES

There is a systemic relationship between cyclonesand fronts, in that the cyclones are usually associatedwith waves along fronts—primarily cold fronts.Cyclones come into being or intensify because pressurefalls more rapidly at one point than it does in thesurrounding area. Cyclogenesis can occur anywhere,but in middle and high latitudes, it is most likely tooccur on a frontal trough. When a cyclone (or simplylow) develops on a front, the cyclogenesis begins at thesurface and develops gradually upward as the cyclonedeepens. The reverse also occurs; closed circulationsaloft sometime work their way downward until theyappear on the surface chart. These cyclones rarelycontain fronts and are quasi-stationary or drift slowlywestward and/or equatorward.

Every front, however, is associated with a cyclone.Fronts move with the counterclockwise flow associatedwith Northern Hemisphere cyclones and clockwisewith the flow of Southern Hemisphere cyclones. Themiddle latitudes are regions where cold and warm airmasses continually interact with each other. Thisinteraction coincides with the location of the polarfront.

When the polar front moves southward, it is usuallyassociated with the development and movement ofcyclones and with outbreaks of cold polar air. Thecyclonic circulation associated with the polar fronttends to bring polar air southward and warm moisttropical air northward.

During the winter months, the warm airflowusually occurs over water and the cold air movessouthward over continental areas. In summer thesituation is reversed. Large cyclones that form on thepolar front are usually followed by smaller cyclonesand are referred to as families. These smaller cyclones

tend to carry the front farther southward. In an idealsituation these cyclones come in succession, causingthe front (in the Northern Hemisphere) to lie in asouthwest to northeast direction.

Every moving cyclone usually has two significantlines of convergence distinguished by thermalproperties. The discontinuity line on the forward side ofthe cyclone where warm air replaces cold air is thewarm front; the discontinuity line in the rear portion ofthe cyclone where cold air displaces warm air is thecold front.

The polar front is subject to cyclonic developmentalong it. When wind, temperature, pressure, and upperlevel influences are right, waves form along the polarfront. Wave cyclones normally progress along the polarfront with an eastward component at an average rate of25 to 30 knots, although 50 knots is not impossible,especially in the case of stable waves. These waves mayultimately develop into full-blown low-pressuresystems with gale force winds. The development of asignificant cyclone along the polar front depends onwhether the initial wave is stable or unstable. Waveformation is more likely to occur on slowly moving orstationary fronts like the polar front than on rapidlymoving fronts. Certain areas are preferred localities forwave cyclogenesis. The Rockies, the Ozarks, and theAppalachians are examples in North America.

Stable Waves

A stable wave is one that neither develops noroccludes, but appears to remain in about the same state.Stable waves usually have small amplitude, weak lowcenters, and a fairly regular rate and direction ofmovement. The development of a stable wave is shownin views A, B, and C of figure 4-21. Stable waves do notgo into a growth and occlusion stage.

4-22

COLDAIR MASS

AG5f0421

POLARFRONT

WARMAIR MASS

A. COLD AND WARM AIR FLOW B. FORMATION C. TYPICAL WAVE

Figure 4-21.—Life cycle of a stable wave cyclone.

Unstable Waves

The unstable wave is by far the more common wavethat is experienced with development along the polarfront. The amplitude of this wave increases with timeuntil the occlusion process occurs. The formation of adeep cyclone and an occluded front breaks up the polarfront. When the occlusion process is complete, thepolar front is reestablished. This process is shown infigure 4-22. Views A through G of figure 4-22, referredto in the next three paragraphs, show the life cycle ofthe unstable wave.

In its initial stage of development, the polar frontseparates the polar easterlies from the mid-latitudewesterlies (view A); the small disturbance caused bythe steady state of the wind is often not obvious on theweather map. Uneven local heating, irregular terrain, or

wind shear between the opposing air currents may starta wavelike perturbation on the front (view B); if thistendency persists and the wave increases in amplitude,a counterclockwise (cyclonic) circulation is set up. Onesection of the front begins to move as a warm frontwhile the adjacent sections begin to move as a coldfront (view C). This deformation is called a frontalwave.

The pressure at the peak of the frontal wave falls,and a low-pressure center is formed. The cycloniccirculation becomes stronger; the wind components arenow strong enough to move the fronts; the westerliesturn to southwest winds and push the eastern part of thefront northward as a warm front; and the easterlies onthe western side turn to northerly winds and push thewestern part southward as a cold front. The cold front ismoving faster than the warm front (view D). When the

4-23

COLD

WARM

COLD

WARM

COLD

FRONTAL

WAVE

WARM

COOL

COLD

WARMWARM

COLD

WARM

COLD

COOL

WARM

COLD

COOL

A

B

C

D

E

F

GAG5f0422

Figure 4-22.—Life cycle of an unstable frontal wave.

cold front overtakes the warm front and closes thewarm sector, an occlusion is formed (view E). This isthe time of maximum intensity of the wave cyclone.

As the occlusion continues to extend outward, thecyclonic circulation diminishes in intensity (thelow-pressure area weakens), and the frontal movementslows down (view F). Sometimes a new frontal wavemay begin to form on the westward trailing portion ofthe cold front. In the final stage, the two fronts becomea single stationary front again. The low center with itsremnant of the occlusion has disappeared (view G).Table 4-2 shows the numerical average life cycle of atypical unstable wave cyclone from initial developmentto cyclolysis. It is only intended to be used as a guide inareas where reports are sparse.

FRONTOGENESIS AND FRONTOLYSIS

LEARNING OBJECTIVE: Describe theconditions necessary for frontogenesis andfrontolysis, and identify the worldfronto-genetical zones.

CONDITIONS NECESSARY FORFRONTOGENESIS

Frontogenesis is the formation of a new front or theregeneration of an old one. Frontogenesis takes placeonly when two conditions are met. First, two air massesof different densities must exist adjacent to one another;and second, a prevailing wind field must exist to bringthem together. There are three basic situations, whichare conducive to frontogenesis and satisfy the two basicrequirements.

The windflow is cross isothermal and flowing fromcold air to warmer air. The flow must be crossisothermal, resulting in a concentration of isotherms(increased temperature gradient). The flow does nothave to be perpendicular; however, the moreperpendicular the cross isothermal flow, the greater theintensity of frontogenesis.

The winds of opposite air masses move toward thesame point or line in that cross-isothermal flow. Aclassic example of this situation is the polar front wherecold polar air moves southward toward warmer

4-24

Time (hours)

Central Pressure (mb)

Direction of Movement(toward)

Speed of Movement(knots)

NE to SE or(quad)

30-35

1,012-1,000

Wave Cyclone

1000-988

NNE to N(arc)

20-25

984-968

N to NNW(arc)

10-15

998-1,004

0-5

Occlusion Mature Occlusion Cyclolysis

0 12-24 24-36 36-72

The symbol indicates that the filling center drifts slowly in a counterclockwise direction alongan approximately circular path about a fixed point.

AG5t0402

Table 4-2.—Numerical Characteristics of the Life Cycle of an Unstable Wave Cyclone

T1

T2

T3

T4

T5

H L

XX

L H

Y

T6

Y

T1T2

T3T4

T5T6

H L

Y

XX

L H

X

T1T2T3T4T5T6

H L

Y

X

L H

A. IDEALLY FRONTOGENETIC B. CRITICALLY FRONTOGENETIC C. IDEALLY FRONTOLYTIC

AG5f0423

Y

Figure 4-23.—Perpendicular deformation field.

temperatures and warm tropical air moves northwardtoward colder temperatures.

The wind flow has formed a deformation field. Adeformation field consists basically of an area of flatpressure between two opposing highs and two opposinglows (also called a COL or saddle). It has two axes thathave their origin at a neutral point in the COL (view Ain fig. 4-23). The y axis, or axis of contraction, liesbetween the high and low that bring the air particlestoward the neutral point. (Note the flow arrows in fig.4-23.) The x axis lies between the high and low that takeair particles away from the neutral point and is knownas the axis of dilation.

The distribution and concentration of isotherms T1through T6 in this deformation field determine whetherfrontogenesis results. If the isotherms form a largeangle with the axis of contraction, frontogenesisresults. If a small angle exists, frontolysis (thedissipation of a front) results. It has been shown that in aperpendicular deformation field, isotherms must forman angle of 450 or less with the axis of dilation forfrontogenesis to occur as shown in views A and B of thefigure. In a deformation field not perpendicular, thecritical angle changes correspondingly as illustrated inviews A and B of figure 4-24. In most cases,frontogenesis occurs along the axis of dilation.Frontogenesis occurs where there is a concentration ofisotherms with the circulation to sustain thatconcentration.

CONDITIONS NECESSARY FORFRONTOLYSIS

Frontolysis, or the dissipation of a front, occurswhen either the temperature difference between the twoair masses disappears or the wind carries the air

particles of the air mass away from each other.Frontolytical processes are more common in theatmosphere than are frontogenetical processes. Thiscomes about because there is no known property of theair, which is conservative with respect to all thephysical or dynamical processes of the atmosphere.

Frontolytical processes are most effective in thelower layers of the atmosphere since surface heatingand turbulent mixing are the most intense of thenonconservative influences on temperature.

For frontolysis to occur, only one of the twoconditions stated above need be met. The simultaneoushappening of both conditions results in more rapidfrontolysis than if only one factor were operative. Theshape and curvature of the isobars also give valuableindications of frontolysis and frontogenesis, and,therefore, possible cyclolysis or cyclogenesis.

On a cold front, anticyclonically curved isobarsbehind the front indicate that the FRONT is slowmoving and therefore exposed to frontogenesis.

Cyclonically curved isobars in the cold air behindthe cold front indicate that the front is fast moving andexposed to frontolysis. On the warm fronts the converseis true.

Anticyclonically curved isobars in advance of thewarm front indicate the front is fast moving andexposed to frontolysis.

With cyclonically curved isobars the warm front isretarded and exposed to frontogenesis.

WORLD FRONTOGENETICAL ZONES

Certain regions of the world exhibit a highfrequency of frontogenesis. These regions are

4-25

T1

T2

T3

T4

T5

H

L

X

X

Y

T6

T1

T2

T3

T4T5

T6

H

L

Y

T1

T2T3

T4T5

T6

H

L

A. FRONTOGENETIC B. CRITICALLY FRONTOGENETIC C. IDEALLY FRONTOLYTIC

AG5f0424

YH

H

Y

X

X

Y

YH

X

X

L

LL

Figure 4-24.—Nonperpendicular deformation field.

coincident with the greatest temperature contrasts. Twoof the most important frontal zones are those over thenorth Pacific and north Atlantic Oceans. In winter, thearctic front, a boundary between polar and arctic air,forms in high latitudes over northwest North America,the north Pacific, and near the Arctic Circle north ofEurope (fig. 4-25). In summer, the arctic front mainlydisappears, except north of Europe. (See fig. 4-26.)

The polar front, on the other hand, is present theyear round, although it is not as intense in the summeras in the winter because of a lessening temperaturecontrast between the opposing air masses. The polarfront forms wherever the wind flow and temperaturecontrast is favorable. Usually this is the boundarybetween tropical and polar air, but it may form betweenmaritime polar and continental polar air. It also mayexist between modified polar air and a fresh outbreak ofpolar air. The polar front is common over NorthAmerica in the continental regions in winter in thevicinity of 50°N latitude.

The polar front in winter is found most frequentlyoff the eastern coasts of continents in areas of 30° to 60°

latitude. It is also found over land; but since thetemperature contrasts are greater between the continentand the oceans, especially in winter, the coastal areasare more favorable for formation and intensification ofthe polar front.

The intertropical convergence zone (ITCZ), thoughnot truly a front but a field of convergence between theopposing trades, forms a third semipermanent frontaltype. This region shows a seasonal variation just as dothe trade winds.

FRONTAL CHARACTERISTICS

LEARNING OBJECTIVE: Describe thefrontal elements and general characteristics offronts.

FRONTAL ELEMENTS

From our previous discussion and definitions offronts, it was implied that a certain geometrical andmeteorological consistency must exist between fronts

4-26

cASOURCE

ARCTIC FRONT

LOWMP

1020

1020

HIGH

HIGHP

OLA

R FRONT

cA AM

SOURCE

LOWARCTIC

FRONT

1000 MP

M CP P

POLAR FRONT (ATLANTIC)

1020HIGH

MT

CP

(I CT Z)

MT

(I CT Z)

MED POLAR FRONT

1020

CSOURCE

TH CT

1020HIGHL

LOW

1020HIGH

MT1010

SO. PACIFIC POLAR FRONT

SO. ATLANTIC POLAR FRONT

MT

1020

HIGH

MT

(ITCZ)

POLAR FRONT

CT

CTLOW

POLAR FRONT

LOWC

SOURCEP

1032

CSOURCE

A

ASIA ARCTIC FRONT

CA

180 150 120 90 60 30 0 30 60 90 120 150 180

75

60

45

30

15

0

15

30

45

60

75

60

45

30

15

0

15

30

45

60

POLAR FRONT (SO. INDIAN OCEAN)

POLARFRONT (PACIFIC)

JANUARYAG5f0425

Figure 4-25.—Chart showing world air masses, fronts and centers of major pressure systems in January.

at adjoining levels. It can also be inferred that the data atno one particular level is sufficient to locate a front withcertainty in every case. We must consider the horizontaland vertical distribution of three weather elements(temperature, wind, and pressure) in a frontal zone.

Temperature

Typical fronts always consist of warm air abovecold air. A radiosonde observation taken through afrontal surface often indicates a relatively narrow layerwhere the normal decrease of temperature with heightis reversed. This temperature inversion is called afrontal inversion; its position indicates the height of thefrontal surface and the thickness of the frontal zoneover the particular station. The temperature increasewithin the inversion layer and the thickness of the layercan be used as a rough indication of the intensity of afront. Strong fronts tend to have a distinct inversion;moderate fronts have isothermal frontal zones; and

weak fronts have a decrease in temperature through thefrontal zone.

Frontal zones are often difficult to locate on asounding because air masses become modified afterleaving their source region and because of turbulentmixing and falling precipitation through the frontalzone. Normally, however, some indication does exist.The degree to which a frontal zone appears pronouncedis proportional to the temperature difference betweentwo air masses.

The primary indication of a frontal zone on a SkewT diagram is a decrease in the lapse rate somewhere inthe sounding below 400 mb. The decrease in lapse ratemay be a slightly less steep lapse rate for a stratum in aweak frontal zone to a very sharp inversion in strongfronts. In addition to a decrease in the lapse rate, there isusually an increase in moisture (a concurrent dew-pointinversion) at the frontal zone. This is especially truewhen the front is strong and abundant cloudiness and

4-27

SOURCE

POLAR FRONTMP

1024

LOWHIGH

CT

180 150 120 90 60 30 0 30 60 90 120 150 180

75

60

45

30

15

0

15

30

45

60

75

60

45

30

15

0

15

30

45

60

POLARFRONT

JULY

MT

1010 MT

MT 1020

1024HIGH

1012

MPLOW

(NO. AM. NO. ATL)

CT

SOURCE

MP CPARCTIC FRONT

POLARFRONT

CP CT

CP

SOURCE

ASIAN POLO

AR

R FNT

CSOURCE

T

LOW

MP

PACIFIC POLAR

FRONT1018

1010

1010

(I C )T Z

CT

SOURCE

MT

E

HIGH1026 AUSTRALIAN POLARFRONT

MP

INDIAN OCEAN P TOLAR OFR N

ESOURCE

MT1020

HIGH

C TSOURCE

(I C )T Z

POLA .R LTFR O.AONT (S )

MP

(I C )T Z

1020

HIGHMT

MP

E

P

C

O(

LAR FRONT SO.PA .)

1010

1010

1005

1000

CP

AG5f0426

Figure 4-26.—Chart showing world air masses, fronts and centers of major pressure systems in July.

precipitation accompany it. View A of figure 4-27shows the height of the inversion in two different partsof a frontal zone, and view B of figure 4-27 shows astrong frontal inversion with a consequent dew-pointinversion.

A cold front generally shows a stronger inversionthan a warm front, and the inversion appears atsuccessively higher levels as the front moves past astation. The reverse is true of warm fronts. Occludedfronts generally show a double inversion. However, asthe occlusion process continues, mixing of the airmasses takes place, and the inversions are wiped out orfuse into one inversion.

It is very important in raob analysis not to confusethe subsidence inversion of polar and arctic air masseswith frontal inversions. Extremely cold continentalarctic air, for instance, has a strong inversion thatextends to the 700-mb level. Sometimes it is difficult tofind an inversion on a particular sounding, though it isknown that a front intersects the column of air over agiven station. This may be because of adiabatic

warming of the descending cold air just under thefrontal surface or excessive local vertical mixing in thevicinity of the frontal zone. Under conditions ofsubsidence of the cold air beneath the frontal surface,the subsidence inversion within the cold air may bemore marked than the frontal zone itself.

Sometimes fronts on a raob sounding, which mightshow a strong inversion, often are accompanied by littleweather activity. This is because of subsidence in thewarm air, which strengthens the inversion. The weatheractivity at a front increases only when there is a netupward vertical motion of the warm air mass.

Wind

Since winds near Earth’s surface flow mainly alongthe isobars with a slight drift toward lower pressure, itfollows that the wind direction in the vicinity of a frontmust conform with the isobars. The arrows in figure4-28 indicate the winds that correspond to the pressuredistribution.

4-28

ATMOSPHERIC SOUNDINGSIN THE COLD AIR MASS

FRONTAL SURFACE

WARM AIR MASS

COLD AIR MASSSURFACEPOSITION

OF FRONT

WARM AIR MASS

FRONTAL ZONE

FRONTAL SLOPE

DEW POINT

TEMPERATURE

ACCUMULATIONOF MOISTURE

HERE

A. HEIGHT AND THICKNESSOF INVERSION INDICATES SLOPE

OF FRONT AND INTENSITY

B. FRONTAL INVERSION

AG5f0427

Figure 4-27.—Inversions.

HIGH

AG5f0428

HIGH

LOW1004

1008

A

HIGH

HIGH

LOWB

1000

1004

HIGH

HIGH

LOWC

1000

1004HIGH

HIGH

LOWD

996

1000

Figure 4-28.—Types of isobars associated with fronts.

From this it can be seen that a front is a wind shiftline and that wind shifts in a cyclonic direction.Therefore, we can evolve the following rule: if youstand with your back against the wind in advance of thefront, the wind will shift clockwise as the front passes.This is true with the passage of all frontal types. Referback to figure 4-22.

NOTE: The wind flow associated with thewell-developed frontal system is shown in figure 4-22,view E. Try to visualize yourself standing ahead of eachtype of front depicted as they move from west to east.The terms backing and veering are often used whendiscussing the winds associated with frontal systems.

BACKING.—Backing is a change in winddirection—counterclockwise in the NorthernHemisphere and clockwise in the SouthernHemisphere. The opposite of backing is veering.

VEERING.—Veering is a change in winddirection—clockwise in the Northern Hemisphere,counterclockwise in the Southern Hemisphere. Theopposite of veering is backing.

The speed of the wind depends upon the pressuregradient. Look at figure 4-28. In view A, the speed isabout the same in both air masses; in views B and C, arelatively strong wind is followed by a weaker wind;and in view D, a weak wind is followed by a strongwind. An essential characteristic of a frontal zone is awind discontinuity through the zone. The windnormally increases or decreases in speed with heightthrough a frontal discontinuity. Backing usually occurswith height through a cold front and veering through awarm front. The sharpness of the wind discontinuity is

proportional to the temperature contrast across the frontand the pressure field in the vicinity of the front (thedegree of convergence between the two air streams).With the pressure field constant, the sharpness of thefrontal zone is proportional to the temperaturediscontinuity (no temperature discontinuity—no front;thus, no wind discontinuity). The classical picture ofthe variation in wind along the vertical through a frontalzone is shown in figure 4-29. An example of a frontalzone and the winds through the frontal zone is shown infigure 4-30.

4-29

AG5f0429

COLDAIR

CO

LD

FR

ON

T

COLDAIR

WARM AIR

WARM

FRONT

N

W

S

E

Figure 4-29.—Vertical distribution of wind direction in the vicinity of frontal surfaces.

AG5f0430

MBS500

600

700

800

900

1000

-30 -20 -10 0

TdT

FRONTALZONE

Figure 4-30.—Distribution of wind and temperature through awarm frontal zone.

On this sounding the upper winds that show thegreatest variation above the surface layer are thosebetween the 800- to 650-mb layers. This indicationcoincides closely with the frontal indications of thetemperature (T) and dew point (Td) curves (see fig.4-30). Since the wind veers with height through thelayer, the front would be warm. The vertical wind shiftthrough a frontal zone depends on the direction of theslope. In cold fronts the wind backs with height, whilein warm fronts the wind veers with height. At thesurface the wind always veers across the front, and theisobars have a sharp cyclonic bend or trough that pointstoward higher pressure. Sometimes the associatedpressure trough is not coincident with the front; in suchcases there may not be an appreciable wind shift acrossthe front—only a speed discontinuity.

Pressure

One of the important characteristics of all fronts isthat on both sides of a front the pressure is higher thanat the front. This is true even though one of the airmasses is relatively warm and the other is relativelycold. Fronts are associated with troughs of lowpressure. (A trough is an elongated area of relativelylow pressure.) A trough may have U-shaped orV-shaped isobars. How the pressure changes with thepassage of a front is of prime importance when you aredetermining frontal passage and future movement.

Friction causes the air (wind) near the ground todrift across the isobars toward lower pressure. Thiscauses a drift of air toward the front from both sides.Since the air cannot disappear into the ground, it mustmove upward. Hence, there is always a net movementof air upward in the region of a front. This is animportant characteristic of fronts, since the lifting of theair causes condensation, clouds, and weather.

GENERAL CHARACTERISTICS OF FRONTS

All fronts have certain characteristics that arecommon and usually predictable for that type of front.Cold frontal weather differs from warm frontalweather, and not every cold front has the same weatherassociated with it. The weather, intensity of theweather, and the movement of fronts are, to a largedegree, associated with the slope of the front.

Frontal Slope

When we speak of the slope of a front, we arespeaking basically of the steepness of the frontalsurface, using a vertical dimension and a horizontal

dimension. The vertical dimension used is normally 1mile. A slope of 1:50 (1 mile vertically for every 50miles horizontally) would be considered a steep slope,and a slope of 1:300 a gradual slope. Factors favoring asteep slope are a large wind velocity difference betweenair masses, small temperature difference, and highlatitude.

The frontal slope therefore depends on the latitudeof the front, the wind speed, and the temperaturedifference between the air masses. Because cold airtends to under run warm air, the steeper the slope, themore intense the lifting and vertical motion of the warmair and, therefore the more intense the weather.

Clouds and Weather

Cloud decks are usually in the warm air massbecause of the upward vertical movement of the warmair. Clouds forming in a cold air mass are caused by theevaporation of moisture from precipitation from theoverlying warm air mass and/or by vertical lifting.Convergence at the front results in a lifting of bothtypes of air. The stability of air masses determines thecloud and weather structure at the fronts as well as theweather in advance of the fronts.

Frontal Intensity

No completely acceptable set of criteria is inexistence as to the determination of frontal intensity, asit depends upon a number of variables. Some of thecriteria that may be helpful in delineating frontalintensity are discussed in the following paragraphs.

TURBULENCE.—Except when turbulence orgustiness may result, weather phenomena are not takeninto account when specifying frontal intensity, becausea front is not defined in terms of weather. A front maybe intense in terms of discontinuity of density across it,but may be accompanied by no weather phenomenaother than strong winds and a drop in temperature. Afront that would otherwise be classified as weak isconsidered moderate if turbulence and gustiness areprevalent along it, and an otherwise moderate front isclassified as strong if sufficient turbulence andgustiness exist. The term gustiness for this purposeincludes convective phenomena such as thunderstormsand strong winds.

TEMPERATURE GRADIENT.—Temperaturegradient, rather than true difference of temperatureacross the frontal surface, is used in defining the frontalintensity. Temperature gradient, when determining

4-30

frontal intensity, is defined as the difference betweenthe representative warm air immediately adjacent to thefront and the representative surface temperature 100miles from the front on the cold air side.

A suggested set of criteria based on the horizontaltemperature gradient has been devised. A weak front isone where the temperature gradient is less than 100Fper 100 miles; a moderate front is where thetemperature gradient is 10 0F to 20 0F per 100 miles;and a strong front is where the gradient is over 20 0F per100 miles.

The 850-mb level temperatures may be used in lieuof the surface temperatures if representative surfacetemperatures are not available and the terrain elevationis not over 3,000 feet. Over much of the western sectionof the United States, the 700-mb level temperatures canbe used in lieu of the surface temperatures.

Speed

The speed of the movement of frontal systems is animportant determining factor of weather conditions.Rapidly moving fronts usually cause more severeweather than slower moving fronts. For example,fast-moving cold fronts often cause severe prefrontalsquall lines that are extremely hazardous to flying. Thefast-moving front does have the advantage of movingacross the area rapidly, permitting the particularlocality to enjoy a quick return of good weather.Slow-moving fronts, on the other hand, may causeextended periods of unfavorable weather. A stationaryfront that may bring bad weather can disrupt flightoperations for several days in succession. The specificcharacteristics of each of the types of fronts is discussedin lessons 3 through 6.

Wind Component

The speed of a front is controlled by a resultantcomponent of wind behind a front. The windcomponent normal to a front is determined by the angleat which the geostrophic winds blow toward the front,resulting in a perpendicular force applied to the back ofthe front. For example, the component of the windnormal to a front that has a geostrophic wind with aperpendicular flow of 30 knots behind the front has a30-knot component. However, a 30-knot geostrophicwind blowing at a 450 angle to the front has only a15-knot component that is normal to or perpendicularto the front. The greater the angle of the wind to thefront, the greater the wind component normal to that

front. The smaller the angle, the less the windcomponent normal to the front.

REVIEW QUESTIONS

Q4-6. What is the definition of a frontal surface?

Q4-7. Where is the frontal zone located?

Q4-8. What is the difference between a stable waveand an unstable wave?

Q4-9. Where does frontogenesis occur?

Q4-10. Where is the polar front normally foundduring the winter?

THE COLD FRONT

LEARNING OBJECTIVE: Describeslow-moving cold fronts, fast-moving coldfronts, secondary cold fronts, and cold frontsaloft.

A cold front is the leading edge of a wedge of coldair that is under running warm air. Cold fronts usuallymove faster and have a steeper slope than other types offronts. Cold fronts that move very rapidly have verysteep slopes in the lower levels and narrow bands ofclouds that are predominant along or just ahead of thefront. Slower moving cold fronts have less steep slopes,and their cloud systems may extend far to the rear of thesurface position of the fronts. Both fast-moving andslow-moving cold fronts may be associated with eitherstability or instability and either moist or dry airmasses.

Certain weather characteristics and conditions aretypical of cold fronts. In general, the temperature andhumidity decrease, the pressure rises, and in theNorthern Hemisphere the wind shifts (usually fromsouthwest to northwest) with the passage of a coldfront. The distribution and type of cloudiness and theintensity and distribution of precipitation dependprimarily on the vertical motion within the warm airmass. This vertical motion is in part dependent upon thespeed of that cold front.

SLOW-MOVING COLD FRONTS (ACTIVECOLD FRONT)

With the slow-moving cold front, there is a generalupward motion of warm air along the entire frontalsurface and pronounced lifting along the lower portionof the front. The average slope of the front is

4-31

approximately 1:100 miles. Near the ground, the slopeis often much steeper because of surface friction.

Figure 4-31 illustrates the typical characteristics inthe vertical structure of a slow-moving cold front. Thelower half shows the typical upper airflow behind thefront, and the upper half shows the accompanyingsurface weather. This is only one typical case. Manyvariations to this model can and do occur in nature. Theslow-moving cold front is an active front because it haswidespread frontal cloudiness and precipitation at andbehind the front caused by actual frontal lifting.

Surface Characteristics

The pressure tendency associated with this type offrontal passage is indicated by either an unsteady or

steady fall prior to frontal passage and then weak risesbehind. Temperature and dew point drop sharply withthe passage of a slow-moving cold front. The windveers with the cold frontal passage and reaches itshighest speed at the time of frontal passage. Isobars areusually curved anticyclonically in the cold air. Thistype of front usually moves at an average speedbetween 10 and 15 knots. Slow-moving cold frontsmove with 100% of the wind component normal to thefront.

Weather

The type of weather experienced with aslow-moving cold front is dependent upon the stabilityof the warm air mass. When the warm air mass is stable,

4-32

200 150 100 50 0 50 100 MILES

PRECIP.AREA

NIMBOSTRATUS

ALTOCUMULUS

POSSIBLECUMULONIMBUS

POINT A

COLD AIR

SUBSIDENCEINVERSION

FRONTALINVERSION

TEMP.CURVE

WIND

ST ST

AS

15,000

10,000

5,000

ALTITUDE FT

CIRRO-STRATUS

CIRRUS

UPPER AIRTROUGH

UPPER

WIN

DFLO

W

AREAOF

CLOUDY

SKIESAND

RAIN

(SURFACE WINDFLOW)POINT A

AG5f0431

0 CO0 CO

WIND

Figure 4-31.—Typical vertical structure of a slow-moving cold front with upper windflow in back of the front.

a rather broad zone of altostratus and nimbostratusclouds accompany the front and extend several hundredmiles behind the front. If the warm air is unstable (orconditionally unstable), thunderstorms andcumulonimbus clouds may develop within the cloudbank and may stretch for some 50 miles behind thesurface front. These cumulonimbus clouds form withinthe warm air mass. In the cold air there may be somestratus or nimbostratus clouds formed by theevaporation of falling rain; but, generally, outside of therain areas, there are relatively few low clouds. This isbecause of the descending motion of the cold air thatsometimes produces a subsidence inversion somedistance behind the front.

The ceiling is generally low with the frontalpassage, and gradual lifting is observed after passage.Visibility is poor in precipitation and may continue tobe reduced for many hours after frontal passage as longas the precipitation occurs. When the cold air behindthe front is moist and stable, a deck of stratus cloudsand/or fog may persist for a number of hours afterfrontal passage. The type of precipitation observed isalso dependent upon the stability and moistureconditions of the air masses.

Upper Air Characteristics

Upper air contours show a cyclonic flow and areusually parallel to the front as are the isotherms. Theweather usually extends as far in back of the front asthese features are parallel to it. When the orientationchanges, this usually indicates the position of theassociated upper air trough. (A trough is an elongatedarea of relatively low pressure.)

The temperature inversion on this type of front isusually well marked. In the precipitation area therelative humidity is high in both air masses. Fartherbehind the front, subsidence may occur, giving asecond inversion closer to the ground.

The wind usually backs rapidly with height (on theorder of some 60 to 70 degrees between 950 and 400mb), and at 500 mb the wind direction is inclined atabout 15 degrees to the front. The wind componentnormal to the front decreases slightly with height, andthe component parallel to the front increases rapidly.

On upper air charts, slow-moving cold fronts arecharacterized by a packing (concentration) ofisotherms behind them. The more closely packed theisotherms and the more nearly they parallel the fronts,the stronger the front.

FAST-MOVING COLD FRONTS (INACTIVECOLD FRONT)

The fast-moving cold front is a very steep front thathas warm air near the surface being forced vigorouslyupward. At high levels, the warm air is descendingdownward along the frontal surface. This front has aslope of 1:40 to 1:80 miles and usually moves rapidly;25 to 30 knots may be considered an average speed ofmovement. They move with 80 to 90 percent of thewind component normal to the front. As a result ofthese factors, there is a relatively narrow but oftenviolent band of weather.

Figure 4-32 shows a vertical cross section of afast-moving cold front with resultant weather. Alsoindicated in the lower half of the diagram is the surfaceweather in advance of the front and the upper airflowabove the front.

If the warm air is moist and unstable, a line ofthunderstorms frequently develops along this front.Sometimes, under these conditions, a line of strongconvective activity is projected 50 to 200 miles ahead ofthe front and parallel to it. This may develop into a lineof thunderstorms called a squall line. On the other hand,when the warm air is stable, an overcast layer ofaltostratus clouds and rain may extend over a large areaahead of the front. If the warm air is very dry, little or nocloudiness is associated with the front. The frontdepicted is a typical front with typical characteristics.

The fast-moving cold front is considered aninactive front because lifting occurs only at and aheadof the front. The lifting is caused by descending airahead of the front and only in part by the frontalsurface.

Surface Characteristics

Pressure tendencies fall ahead of the front withsudden and strong rises after frontal passage. If a squallline lies some distance ahead of the front, there may bea strong rise associated with its passage and a shift inthe wind. However, after the influence of the squall linehas passed, winds back to southerly and pressures leveloff. The temperature falls in the warm air just ahead ofthe front. This is caused by the evaporation of fallingprecipitation. Rapid clearing and adiabatic warmingjust behind the front tend to keep the cold airtemperature near that of the warm air. An abrupttemperature change usually occurs far behind the frontnear the center of the high-pressure center associatedwith the cold air mass. The dew point and wind

4-33

directions are a good indication of the passage of afast-moving cold front. The wind veers with frontalpassage and is strong, gusty, and turbulent for aconsiderable period of time after passage. The dewpoint decreases sharply after frontal passage.

Weather

Cumulonimbus clouds are observed along and justahead of the surface front. Stratus, nimbostratus, andaltostratus may extend ahead of the front in advance ofthe cumulonimbus and may extend as much as 150

4-34

200 150 100 50 0 50 100

PRECIP.AREA

CUMULONIMBUS

ALTOCUMULUS

0 CO

CIRROSTRATUS

COLDAIR

SUBSIDENCEINVERSION

FRONTALINVERSION

CUMULUS

WIND

Ns

15,000

10,000

5,000

ALTITUDE FT

CIRRUS

AG5f0432

STRATOCUMULUS

20,000

25,000

WIND

0O

UPPERWIND-FLOW

OR

OR

GENERALLY

CLEAR

SKIES

Figure 4-32.—Typical vertical structure of a fast-moving cold front with upper windflow across the front.

miles ahead of the front. These clouds are all found inthe warm air. Generally, unless the cold air is unstableand descending currents are weak, there are few cloudsin the cold air behind the front. Showers andthunderstorms occur along and just ahead of the front.The ceiling is low only in the vicinity of the front.Visibility is poor during precipitation but improvesrapidly after the frontal passage.

Upper Air Characteristics

Because of the sinking motion of the cold airbehind the front and the resultant adiabatic warming,the temperature change across the front is oftendestroyed or may even be reversed. A sounding taken inthe cold air immediately behind the surface frontindicates only one inversion and an increase in moisturethrough the inversion. Farther back of the front, adouble inversion structure is evident. The lowerinversion is caused by the subsidence effects in the coldair. This is sometimes confusing to the analyst becausethe subsidence inversion is usually more marked thanthe frontal inversion and may be mistaken for thefrontal inversion.

In contrast to the slow-moving cold front, the windabove the fast-moving cold front exhibits only a slightbacking with height of about 20 degrees between 950and 400 mb; the wind direction is inclined toward thefront at an average angle of about 45 degrees. The windcomponents normal and parallel to the front increasewith height; the wind component normal to the frontexceeds the mean speed of the front at all levels abovethe lowest layers. On upper air charts, the isotherms areNOT parallel to the front. Instead they are at an angle ofabout 30 degrees to the front, usually crossing the coldfront near its junction with the associated warm front.

SECONDARY COLD FRONTS

Sometimes there is a tendency for a trough of lowpressure to form to the rear of a cold front, and asecondary cold front may develop in this trough.Secondary cold fronts usually occur during outbreaksof very cold air behind the initial outbreak. Secondarycold fronts may follow in intervals of several hundredmiles to the rear of the rapidly moving front. When asecondary cold front forms, the primary front usuallytends to dissipate and the secondary front then becomesthe primary front. Secondary fronts usually do notoccur during the summer months because there is rarelyenough temperature discontinuity.

COLD FRONTS ALOFT

There are two types of upper cold fronts. One is theupper cold front associated with the warm occlusionthat is discussed later in this unit. The other occursfrequently in the areas just east of mountains in winter.This cold front aloft is associated with mP air crossingthe mountains behind a cold front or behind a coldtrough aloft and a very cold layer of continental polarair lying next to the ground over the area east of themountains. The area east of the Rocky Mountains is onesuch area in the United States. When warm maritimetropical air has moved northward from the Gulf ofMexico and has been forced aloft by the cold cP air, andcool mP air flows over the mountains, it forces its wayunder the warm mT air aloft. The resulting front thenflows across the upper surface of the colder cP air justas if it were the surface of the ground. All frontalactivity in this case takes place above the top of the cPlayer. Figure 4-33 shows an example of this type offront and the synoptic structure. Weather from cold

4-35

FRONTALZONE

AG5f0433

WEST

COOLmP

WARMmT

cPVERY COLD

ROCKYMOUNTAINS

EAST

Figure 4-33.—Cold front aloft.

fronts aloft can produce extensive cloud decks andblizzard conditions for several hundred miles over themid western plains.

INSTABILITY AND SQUALL LINES

The terms instability line and squall line aresynonymous with violent winds, heavy rain, lightning,thunder, hail, and tornadoes. The terms are often usedinterchangeably and are incorrectly applied to anysevere weather phenomena that moves through aregion. However, there is a difference between aninstability line and a squall line.

Instability Line

An instability line is any nonfrontal line or band ofconvective activity. This is a general term and includesthe developing, mature, and dissipating stages of theline of convective activity. However, when the maturestage consists of a line of active thunderstorms, it isproperly termed a squall line. Therefore, in practice, theinstability line often refers only to the less activephases.

Squall Line

A squall line is a nonfrontal line or band of activethunderstorms (with or without squalls). It is themature, active stage of the instability line. From thesedefinitions, instability and squall lines are air massphenomenon because they are both nonfrontaloccurrences. However, they are frequently associatedwith the fast-moving cold front.

NOTE: The term instability line is the moregeneral term and includes the squall line as a specialcase.

Prefrontal Squall Lines

A prefrontal squall line is a squall line located inthe warm sector of a wave cyclone. They form about 50to 300 miles in advance of fast-moving cold fronts andare usually oriented roughly parallel to the cold front.They move in about the same direction as the cold front;however, their speed is, at times, faster than the coldfront. You can roughly compute the direction and speedby using the winds at the 500-mb level. Squall linesgenerally move in the direction of the 500-mb windflow and at approximately 40% of the wind speed.

FORMATION.—There are several theories on thedevelopment of prefrontal squall lines. A generally

accepted theory is that as thunderstorms develop alongthe fast-moving front, large quantities of cold air fromaloft descend in downdrafts along the front and form awedge of cold air ahead of the front. The wedge of coldair then serves as a lifting mechanism for the warm,moist, unstable air; and a line of thunderstormsdevelops several miles in advance of the front. Since thethunderstorms form within the air mass and not alongthe front, the squall line is considered as air massweather (fig. 4-34). In the United States, squall linesform most often in spring and summer. They arenormally restricted to the region east of the RockyMountains with a high frequency of occurrence in thesouthern states.

WEATHER.—Squall-line weather can beextremely hazardous. Its weather is usually more severethan the weather associated with the cold front behindit; this is because the moisture and energy of the warmair mass tends to be released at the squall line prior tothe arrival of the trailing cold front. Showers andthunderstorms (sometimes tornadoes) occur along thesquall line, and the wind shifts cyclonically with theirpassage (fig. 4-35). However, if the zone is narrow, thewind shift may not be noticeable on surface charts.There is generally a large drop in temperature becauseof the cooling of the air by precipitation. Pressure risesafter the passage of the squall line, and, at times, a

4-36

SQUALL LINE DEVELOPMENT

AG5f0434

COLDFRONT

DOWNDRAFTS

UNSTABLEWARM AIR

COLD FRONT

DOWNDRAFTS

UNSTABLEWARM AIR

SQUALL LINE

50 TO 100 MILES

COLD FRONT

Figure 4-34.—Prefrontal squall line development.

micro-high (small high) may form behind it. Afterpassage of the squall line, the wind backs to southerlybefore the cold frontal passage. When the squall linedissipates, severe weather may develop along thefast-moving cold front.

Turbulence is severe in the squall-linethunderstorms because of violent updrafts anddowndrafts. Above the freezing level, icing may occur.Hail is another possibility in the squall-linethunderstorm and can do extensive structural damage toan aircraft. Under the squall line, ceiling and visibilitymay be reduced because of heavy rain showers. Fog is arare occurrence because of the strong wind and gusts,but it may be found in isolated cases. Tornadoesfrequently occur with squall lines when the warm airmass is extremely unstable.

Great Plains Squall Lines

Not all instability lines that reach the mature orsquall-line stage develops in advance of a fast-movingcold front. The Great Plains region of the United Stateshas a high frequency of these squall lines. The GreatPlains type of squall lines also develop in warm, moist,unstable air masses. The necessary lifting or triggermay be supplied by intense thermal heating, orographiclifting, or convergent winds associated with alow-pressure area.

FORMATION.—The Great Plains squall lineforms when an extremely unstable conditiondevelops—normally in spring and summer. Extremelyunstable conditions exist when moist mP air cools inthe upper levels because of the evaporation of falling

precipitation. This cooler air aloft then moves overwarm moist mT air (or even warm, moist, highlymodified mP air) at the surface. If a sufficient triggersuch as a steep lapse rate of a lifting mechanism exists,this extremely unstable situation rapidly develops into asquall line.

WEATHER.—The weather associated with theGreat Plains squall line is the same as that found withthe prefrontal squall line. Because of the extremeinstability, tornadoes are a common occurrence.

REVIEW QUESTIONS

Q4-11. What is the pressure tendency with thepassage of a slow moving cold front?

Q4-12. What is the normal slope of a fast moving coldfront?

Q4-13. Where do prefrontal squall lines normallyform?

THE WARM FRONT

LEARNING OBJECTIVE: Describe thecharacteristics and weather of warm fronts atthe surface and aloft.

A warm front is the line of discontinuity where theforward edge of an advancing mass of relatively warmair is replacing a retreating relatively colder air mass.The slope of the warm front is usually between 1:100and 1:300, with occasional fronts with lesser slopes.Therefore, warm fronts have characteristically shallowslopes caused by the effect of surface friction thatretards the frontal movement near the ground.

SURFACE CHARACTERISTICS

Warm fronts move slower than cold fronts. Theiraverage speed is usually between 10 and 20 knots. Theymove with a speed of 60 to 80 percent of the componentof the wind normal to the front in the warm air mass.

The troughs associated with warm fronts are not aspronounced as those with cold fronts and sometimesmake location difficult on the surface chart. Thepressure tendency ahead of the front is usually a rapidor unsteady fall with a leveling off after frontal passage.A marked decrease in isallobaric gradient is noticed inthe warm sector except when rapid deepening is takingplace. The wind increases in velocity in advance ofwarm fronts because of an increase in pressure gradientand reaches a maximum just prior to frontal passage.The wind veers with frontal passage, usually from a

4-37

AG5f0435

Figure 4-35.—Typical isobaric pattern associated with aprefrontal squall line.

southeasterly direction to a southwesterly directionbehind the front. This shift is not as pronounced as withthe cold front.

Temperature generally is constant or slowly risingin advance of the front until the surface front passes, atwhich time there is a marked rise. This rise is dependentupon the contrast between the air masses. Dew pointusually increases slowly with the approach of the frontwith a rapid increase in precipitation and fog areas. Ifthe warm sector air is maritime tropical, the dew pointshows a further increase.

WEATHER

A characteristic phenomenon of a typical warmfront is the sequence of cloud formations (fig. 4-36).They are noticeable in the following order: cirrus,

cirrostratus, altostratus, nimbostratus, and stratus. Thecirrus clouds may appear 700 to 1,000 miles or moreahead of the surface front, followed by cirrostratusclouds about 600 miles ahead of the surface front andaltostratus about 500 miles ahead of the surface front.

Precipitation in the form of continuous orintermittent rain, snow, or drizzle is frequent as far as300 miles in advance of the surface front. Surfaceprecipitation is associated with the nimbostratus in thewarm air above the frontal surface and with stratus inthe cold air. However, when the warm air isconvectively unstable, showers and thunderstorms mayoccur in addition to the steady precipitation. This isespecially true with a cyclonic flow aloft over the warmfront. Fog is common in the cold air ahead of a warmfront.

4-38

AG5f0436

STABLEWARM

AIR30,000

20,000

10,000

SFC

0 CO

S C

N S

A S

C S

C I

A S

S TF O G , R A I N A N D L O W N I M B O S T R A T U S

200 100 0 100 200 300 400 500 600

SCSC 0 CO

C IC S

A S

N S

C B

UNSTABLEWARM

AIR

30,000

20,000

10,000

SFC

0 CO

200 100 0 100 200 300 400 500 600

L I G H T R A I N A N D S H O W E R S

N SS TS H O W E R S

A C

A S

SC

0 CO

Figure 4-36.—Vertical cross section of a warm front with stable and unstable air.

Clearing usually occurs after the passage of a warmfront, but under some conditions drizzle and fog mayoccur within the warm sector. Warm fronts usuallymove in the direction of the isobars of the warm sector;in the Northern Hemisphere this is usually east tonortheast.

The amount and type of clouds and precipitationvary with the characteristics of the air masses involvedand depending on whether the front is active or inactive.Generally, with warm fronts, an increase of the windcomponent with height perpendicular to the front givesan active front. This produces strong overrunning andpronounced prefrontal clouds and precipitation.Inactive fronts, characterized by broken cirrus andaltocumulus, are produced by a decrease with height ofthe wind component perpendicular to the front.

When the overrunning warm air is moist and stable,nimbostratus clouds with continuous light to moderateprecipitation are found approximately 300 miles aheadof the front. The bases of the clouds lower rapidly asadditional clouds form in the cold air under the frontalsurface. These clouds are caused by evaporation of thefalling rain. These clouds are stratiform when the coldmass is stable and stratocumulus when the cold air isunstable.

When the overrunning air is moist and unstable,cumulus and cumulonimbus clouds are frequentlyimbedded in the nimbostratus and altostratus clouds. Insuch cases, thunderstorms occur along with continuousprecipitation. When the overrunning warm air is dry, itmust ascend to relatively high altitudes beforecondensation can occur. In these cases only high andmiddle clouds are observed. Visibility is usually goodunder the cirrus and altostratus clouds. It decreasesrapidly in the precipitation area. When the cold air isstable and extensive, fog areas may develop ahead ofthe front, and visibility is extremely reduced in thisarea.

UPPER AIR CHARACTERISTICS

Warm fronts are usually not as well defined as coldfronts on upper air soundings. When the front is strongand little mixing has occurred, the front may show awell-marked inversion aloft. However, mixing usuallyoccurs and the front may appear as a rather broad zonewith only a slight change in temperature. Quitefrequently there may be two inversions—one caused bythe front and the other caused by turbulence. Isothermsare parallel to the front and show some form of packingahead of the front. The stronger the packing, the more

active the front. The packing is not as pronounced aswith the cold front.

WARM FRONTS ALOFT

Warm fronts aloft seldom occur, but generallyfollow the same principles as cold fronts aloft. One casewhen they do occur is when the very cold airunderneath a warm front is resistant to displacementand may force the warm air to move over a thinningwedge with a wave forming on the upper surface. Thisgives the effect of secondary upper warm fronts andmay cause parallel bands of precipitation at unusualdistances ahead of the surface warm front. Warm airadvection is more rapid and precipitation is heaviestwhere the steeper slope is encountered. Pressure fallsrapidly in advance of the upper warm front and levelsoff underneath the horizontal portion of the front. Whena warm front crosses a mountain range, colder air mayoccur to the east and may move along as a warm frontaloft above the layer of cold air. This is common when awarm front crosses the Appalachian Mountains inwinter.

REVIEW QUESTIONS

Q4-14. What is the average speed of a warm front?

Q4-15. What cloud types, and in what order usuallyform in advance of a warm front?

THE OCCLUDED FRONTS

LEARNING OBJECTIVE: Describe theformation, structure, and characteristics of coldand warm air occluded fronts.

An occluded front is a composite of two fronts.They form when a cold front overtakes a warm frontand one of these two fronts is lifted aloft. As a result, thewarm air between the cold and warm front is shut off.An occluded front is often referred to simply as anocclusion. Occlusions may be either of the cold type orwarm type. The type of occlusion is determined by thetemperature difference between the cold air in advanceof the warm front and the cold air behind the cold front.

A cold occlusion forms when the cold air inadvance of a warm front is warmer than the cold air tothe rear of the cold front. The overtaking cold airundercuts the cool air in advance of the warm front.This results in a section of the warm front being forcedaloft. A warm occlusion forms when the air in advanceof the warm front is colder than the air to the rear of thecold front. When the cold air of the cold front overtakes

4-39

the warm front, it moves up over this colder air in theform of an upper cold front.

The primary difference between a warm and a coldtype of occlusion is the location of the associated upperfront in relation to the surface front (fig. 4-37). In awarm type of occlusion, the upper cold front precedesthe surface-occluded front by as much as 200 miles. Inthe cold type of occlusion the upper warm front followsthe surface-occluded front by 20 to 50 miles.

Since the occluded front is a combination of a coldfront and a warm front, the resulting weather is that ofthe cold front’s narrow band of violent weather and thewarm front’s widespread area of cloudiness andprecipitation occurring in combination along theoccluded front. The most violent weather occurs at theapex or tip of the occlusion. The apex is the point on thewave where the cold front and warm front meet to startthe occlusion process.

COLD OCCLUSIONS

A cold occlusion is the occlusion that forms when acold front lifts the warm front and the air masspreceding the front (fig. 4-38). The vertical andhorizontal depiction of the cold occlusion is shown infigure 4-39. Cold occlusions are more frequent thanwarm occlusions. The lifting of the warm front as it isunderrun by the cold front implies existence of an upperwarm front to the rear of the cold occlusion; actuallysuch a warm front aloft is rarely discernible and isseldom delineated on a surface chart.

Most fronts approaching the Pacific coast of NorthAmerica from the west are cold occlusions. In winterthese fronts usually encounter a shallow layer ofsurface air near the coastline (from about Oregonnorthward) that is colder than the leading edge of coldair to the rear of the occlusion. As the occluded frontnears this wedge of cold air, the occlusion is forcedaloft and soon is no longer discernible on a surfacechart. The usual practice in these cases is to continue todesignate the cold occlusion as though it were a surfacefront because of the shallowness of the layer over whichit rides. As the occlusion crosses over the mountains, iteventually shows up again on a surface analysis.

The passage of the cold type of occlusion over thecoastal layer of colder air presents a difficult problem ofanalysis in that no surface wind shift ordinarily occursat the exact time of passage. However, a line of stations

4-40

OCCLUD

ED

FR

ON

T

WA

RM

FRONTCOLDFRONT

COLDAIR

COOLAIR

APEX

WARMAIR

UPPER WARM FRONT

O

N

C

O

C

R

L

F

UD

ED

T

WARM

FRONTCOLDFRONT

COOLAIR

APEX

COLDAIR

UPPER COLD FRONT

WARMAIR

A. COLD TYPE OF OCCLUSION B. WARM TYPE OF OCCLUSION

AG5f0437

Figure 4-37.—Sketch of occlusions (in the horizontal) and associated upper fronts.CO

LD-FRONT

SUR

FACE

WARM-FRONT

SURFACE

COLD AIR COOL AIR

WARM AIR

COLD TYPE OCCLUSIONAG5f0438

Figure 4-38.—Vertical cross section of a cold type of occlusion.

reporting surface-pressure rises is the best criterion ofits passage. This should be verified by reference toplotted raob soundings where available. When a Pacificcold occlusion moves farther inland, it sometimesencounters colder air of appreciable depth over thePlateau or Western Plains areas; in this case, it shouldbe redesignated as an upper cold front.

Surface Characteristics

The occlusion lies in a low-pressure area; and in thelatter stages, a separate low center may form at the tip ofthe occlusion, leaving another low-pressure cell nearthe end of the occlusion. The pressure tendency acrossthe cold occluded front follows closely with thoseoutlined for cold fronts; that is, they level off, or moreoften, rapid rises occur after the passage of theoccluded front.

Weather

In the occlusion’s initial stages of development, theweather and cloud sequence ahead of the occlusion isquite similar to that associated with warm fronts;however, the cloud and weather sequence near thesurface position of the front is similar to that associatedwith cold fronts. As the occlusion develops and thewarm air is lifted to higher and higher altitudes, thewarm front and prefrontal cloud systems disappear. Theweather and cloud systems are similar to those of a coldfront. View A of figure 4-39 shows the typical cloudand weather pattern associated with the cold occlusion.Most of the precipitation occurs just ahead of theocclusion. Clearing behind the occlusion is usuallyrapid, especially if the occlusion is in the advancedstage. Otherwise, clearing may not occur until after thepassage of the warm front aloft.

4-41

WARM AIR

COLD FRONT

COLD AIR

FLOW

WARM FRONT

COOL AIR

ALTOSTRATUS

CIRRUS

STRATOCUMULUS

NIMBOSTRATUS

STRATOCUMULUS

RAIN

250 200 150 100 150 0 50 100 150

DISTANCE MILES

COLD AIR COOL AIR

AA'

ISO

BA

RS

B

A

WARM AIR

AG5f0439

ALTOCUMULUS

Figure 4-39.—Cold front type of occlusion. (A) Vertical structure through points A and A'; (B) horizontal structure.

Upper Air Characteristics

If only one upper air sounding were taken so that itintersected either the cold or warm front, the soundingwould appear as a typical warm or cold front sounding.However, if the sounding were taken so that itintersected both the cold and warm air, it would showtwo inversions.

The occlusion may appear on some upper aircharts. It usually appears on the 850-mb chart, butrarely on the 700-mb chart. As the two air masses arebrought closer together and as the occlusion processbrings about gradual disappearance of the warm sector,the isotherm gradient associated with the surface frontweakens. The degree of weakening depends on thehorizontal temperature differences between the cold airto the rear of the cold front and that ahead of the warmfront. The angle at which the isotherms cross thesurface position of the occluded fronts becomes greateras the temperature contrast between the two cold airlasses decreases. A typical illustration of the isothermsshows a packing of isotherms in the cold mass behindthe cold front and less packing in the cool mass inadvance of the warm front. A warm isotherm ridgeprecedes the occlusion aloft.

WARM OCCLUSIONS

A warm occlusion is the occlusion that forms whenthe overtaking cold front is lifted by overrunning thecolder retreating air associated with the warm front.This is shown in figure 4-40. The warm occlusionusually develops in the Northern Hemisphere whenconditions north and of ahead of the warm front aresuch that low pa temperatures exist north of the warmfront. This usually occurs along the west coasts ofcontinents when a relatively cool maritime cold front

overtakes a warm front associated with a very coldcontinental air mass of high pressure situated over thewestern portion of the continent. The cold front thencontinues as an upper cold front above the warm frontsurface. The occlusion is represented as a continuationof the warm front. The cold front aloft is usuallyrepresented on all surface charts. Figure 4-41 depicts atypical warm type of occlusion in both the vertical andhorizontal.

Surface Characteristics

The warm type of occlusion has the same type ofpressure pattern as the cold type of occlusion. The mostreliable identifying characteristics of the upper frontare a line of marked cold frontal precipitation andclouds ahead of the occluded front, a slight but distinctpressure trough and a line of pressure-tendencydiscontinuities.

NOTE: The pressure tendency shows a steady fallahead of the upper cold front and, with passage, aleveling off for a short period of time. Another slightfall is evident with the approach of the surface positionof the occlusion. After passage the pressure shows asteady rise.

The pressure trough is often more distinct with theupper front than with the surface front.

Weather

The weather associated with warm front occlusionshas the characteristics of both warm and cold fronts.The sequence of clouds ahead of the occlusion issimilar to the sequence of clouds ahead of a warm front;the cold front weather occurs near the upper cold front.If either the warm or cool air that is lifted is moist andunstable, showers and sometimes thunderstorms maydevelop. The intensity of the weather along the upperfront decreases with distance from the apex. Weatherconditions change rapidly in occlusions and are usuallymost severe during the initial stages. However, whenthe warm air is lifted to higher and higher altitudes, theweather activity diminishes. When showers andthunderstorms occur, they are found just ahead and withthe upper cold front. Normally, there is clearingweather after passage of the upper front, but this is notalways the case.

Upper Air Characteristics

Upper air soundings taken through either frontshow typical cold or warm front soundings. Those

4-42

COLD-FRONT

SURFACE WARM-FRONT

SURFACE

COLD AIR COLDER AIR

WARM AIR

WARM TYPE OF OCCLUSION AG5f0440

Figure 4-40.—Vertical cross-section of a warm type ofocclusion.

taken that intersect both fronts show two inversions.The warm type of occlusion (like the cold type) appearson upper air charts at approximately the same pressurelevel. However, one distinct difference does appear inthe location of the warm isotherm ridge associated withocclusions. The warm isotherm ridge lies just to the rearof the occlusion at the peak of its development.

REVIEW QUESTIONS

Q4-16. What is the difference between warm and coldocclusions?

Q4-17. Where does the most violent weather occurwith the occlusion?

THE QUASI-STATIONARY FRONT

LEARNING OBJECTIVE: Describe thecharacteristics of stable and unstablequasi-stationary fronts.

A quasi-stationary front, or stationary front as it isoften called, is a front along which one air mass is notappreciably replacing another air mass. A stationaryfront may develop from the slowing down or stoppingof a warm or a cold front. When this front forms, theslope of the warm or cold front is initially very shallow.The dense cold air stays on the ground, and the warmair is displaced slowly upward. The front slows or stopsmoving because the winds behind and ahead of thefront become parallel to the stationary front. It is quite

4-43

WARM AIR

COOL AIR

FLOW

COLD AIR

CUMULONIMBUS

CIRRUS

STRATOCUMULUS OR STRATUS

NIMBOSTRATUS

STRATOCUMULUS

RAIN AND FOG

250200150100

A

DISTANCE MILES

COLD AIR

COOL AIR

A

A'

ISO

BA

RS

B

A

WARM AIR

AG5f0441

OCCLUDED FRONT

500

A'

Figure 4-41.—Illustration of warm type of occlusion. (A) Vertical structure through points A and A'; (B) Horizontal structure.

unusual for two masses of different properties to be sideby side without some movement, so the term stationaryis a misnomer. Actually the front, or dividing linebetween the air masses, is most likely made up of smallwaves undulating back and forth; hence the termquasi-stationary. The important thing is that the front isnot making any appreciable headway in any onedirection. A front moving less than 5 knots is usuallyclassified as a stationary front.

CHARACTERISTICS

When a front is stationary, the whole cold air massdoes not move either toward or away from the front. Interms of wind direction, this means that the wind abovethe friction layer blows neither toward nor away fromthe front, but parallel to it. The wind shift across thefront is usually near 180 degrees. It follows that theisobars, too, are nearly parallel to a stationary front.This characteristic makes it easy to recognize astationary front on a weather map.

STABLE STATIONARY FRONT

There is frictional inflow of warm air toward astationary front causing a slow upglide of air on the

frontal surface. As the air is lifted to and beyondsaturation, clouds form in the warm air above the front.If the warm air in a stationary front is stable and theslope is shallow, the clouds are stratiform. Drizzle maythen fall; and as the air is lifted beyond the freezinglevel, icing conditions develop and light rain or snowmay fall. At very high levels above the front, ice cloudsare present. (See fig. 4-42).

If, however, the slope is steep and significant warmair is being advected up the frontal slope, stratiformclouds with embedded showers result (view B of fig.4-42). Slight undulation or movement of thequasi-stationary front toward the warm air mass adds tothe amount of weather and shower activity associatedwith the front.

UNSTABLE STATIONARY FRONT

If the warm air is conditionally unstable, the slopeis shallow, and sufficient lifting occurs, the clouds arethen cumuliform or stratiform with embedded toweringcumulus. If the energy release is great (warm, moist,unstable air), thunderstorms result. Within the cold airmass, extensive fog and low ceiling may result if thecold air is saturated by warm rain or drizzle falling

4-44

STRATUS

COLD AIR SCUD

WARM AIR

A. SHALLOW STATIONARY FRONT

0 CO

NIMBOSTRATUS

STRATUS ORSTRATOCUMULUS

WARMAIR

0 CO

0 CO

SCUD

COLDAIR

0 CO

B. STEEP STATIONARY FRONTAG5f0442

Figure 4-42.—Types of stable stationary fronts.

through it from the warm air mass above. If thetemperature is below 0°C, icing may occur; butgenerally it is light (view A of fig. 4-43). The shallowslope of an unstable stationary front results in a verybroad and extensive area of showers, fog, and reducedvisibility.

If the slope of an unstable stationary front is steepand sufficient warm air is advected up the slope or thefront moves slowly toward the warm air mass, violentweather can result (view B of fig. 4-43). Heavy rain,severe thunderstorms, strong winds, and tornadoes areoften associated with this front. The width of the bandof precipitation and low ceilings vary from 50 miles toabout 200 miles, depending upon the slope of the frontand the temperatures of the air masses. One of the mostannoying characteristics of a stationary front is that itmay greatly hamper and delay air operations bypersisting in the area for several days.

REVIEW QUESTIONS

Q4-18. When a quasi-stationary front moves, if itdoes, what is the normal speed?

Q4-19. What type of weather is normally associatedwith an unstable stationary front?

MODIFICATIONS OF FRONTS

LEARNING OBJECTIVE: Describe howfronts are modified by their movement,orographic features, and underlying surfaces.

The typical fronts we have just covered can and doundergo modifications that strengthen or weaken them.Such things as frontal movement, orographic effects,and the type of surface the fronts encounter contributeto the modification of fronts.

4-45

SCATTEREDTHUNDERSTORMS

COLD AIR

A. SHALLOW STATIONARY FRONT

0 CO

AG5f0443

WARM AIR

TOWERINGCUMULUS

STRATOCUMULUS

0 CO

SCUD

LINE SQUALLALTOCUMULUS

STRATUS ORSTRATOCUMULUS

COLD AIRWARM

AIR

0 CO

0 CO

SCUD

B. STEEP STATIONARY FRONT

Figure 4-43.—Types of unstable stationary fronts.

EFFECTS CAUSED BY MOVEMENT

The weather is greatly affected by the movement offrontal systems. From the time the front develops untilit passes out of the weather picture, it is watchedclosely. The speed of the movement of frontal systemsis an important determining factor of weatherconditions. Rapidly moving fronts usually cause moresevere weather than slower moving fronts. Fast-movingcold fronts often cause severe prefrontal squall linesthat are extremely hazardous to flying. The fast-movingfront does have the advantage of moving across the arearapidly, permitting the particular locality to enjoy aquick return of good weather. Slow-moving fronts, onthe other hand, may cause extended periods ofunfavorable weather. A stationary front may bring badweather and can disrupt flight operations for severaldays if the frontal weather is sitting over your station.

Knowledge of the speed of the frontal system isnecessary for accurate forecasting. If the front has asomewhat constant speed, it makes your job and theforecaster’s job comparatively easy. However, if thespeed is erratic or unpredictable, you may err as far as

time and severity are concerned. If a front wasultimately forecast to pass through your station andinstead becomes stationary or dissipates, the stationforecast will be a total bust.

OROGRAPHIC EFFECTS

Mountain ranges affect the speed, slope, andweather associated with a front. The height andhorizontal distance of the mountain range along withthe angle of the front along the mountain range are theinfluencing factors. Mountain ranges can affect coldfronts, warm fronts, and occluded fronts differently.

Cold Fronts

As a cold front approaches a mountain range, thesurface portion of the front is retarded and the upperportion pushes up and over the mountain. On thewindward side of the mountain, warm air is pushed upalong the mountain slope because of the additional liftof a now steeper frontal slope and the mountain itself(view A of fig. 4-44). After the front passes the crest ofthe mountain, the air behind the front commences to

4-46

COLD

A

WARM

CLEARING

B

COLD WARM

COLDER

C

EXTREMEINSTABILITY

STAGNANT WARM AIR

AG5f0444

Figure 4-44.—Orographic effects on a cold front.

flow down the leeward side of the range. The warmerair on the leeward side of the mountain is displaced bythe colder air mass. As this cold air descends theleeward side of the mountain, the air warmsadiabatically (view A of fig. 4-44) and clearing occurswithin it. However, since the cold air is displacingwarm air, typical cold frontal clouds and precipitationmay occur within the warm air if the warm air issufficiently moist and conditionally unstable. In somecases maritime polar air that has crossed the Rockies isless dense than maritime tropical air from the Gulf ofMexico that may lie just east of the mountains. If themaritime polar air is moving with a strong westerlywind flow and the maritime tropical air is moving witha strong southerly wind flow, the maritime polar airmay overrun the maritime tropical air. This results inextremely heavy showers, violent thunderstorms, andpossible tornadoes.

If COLDER stagnant air lies to the leeward side ofthe mountain range, the cold front passing over themountain range does not reach the surface but travels as

an upper cold front (view B of fig. 4-44). Under thiscondition, frontal activity is at a minimum. Thissituation does not continue indefinitely; either thestagnant air below mixes with the air above or the uppercold front breaks through to the ground when thestagnant surface air has warmed sufficiently. Then thefront returns to a normal classic front and begins to liftthe now warm air. This ultimately results in thedevelopment of thunderstorms and squall lines (view Cof fig. 4-45). In the summer, this occurs frequently inone form along the eastern United States. When a coldsea breeze occurs and a cold front crosses theAppalachian Mountains, the associated cold wedge ofon-shore flow forces the warm air in advance of thecold front aloft, producing intense thunderstormactivity.

Generally, the area of precipitation is widened as acold front approaches a mountain range. There is anincrease in the intensity of the precipitation and cloudarea on the windward side of the range and a decreaseon the leeward side.

4-47

A.

WARM

FRONTAL SURFACE

B.

COLD

WARM

C.

AG5f0445

FRONTALSURFACE

FRONTALSURFACE

WARM

COLD

COLD

COLD

WARM

WARM

FRONTAL SURFACE

D.

Figure 4-45.—Orographic effects on a warm front.

Warm Fronts

When a warm front approaches a mountain range,the upper section of the frontal surface is above theeffects of the mountain range and does not come underits influence (view A of fig. 4-45). As the lower portionof the frontal surface approaches the range, theunderlying cold wedge is cut off, forming a more or lessstationary front on the windward side of the range. Theinclination of the frontal surface above the rangedecreases and becomes more horizontal near themountain surfaces, but the frontal surface maintains itsoriginal slope at higher altitudes (view B of fig. 4-45).While the stationary front on the windward side of therange may be accompanied by prolonged precipitation,the absence of ascending air on the leeward side of therange causes little or no precipitation. The warm airdescending the leeward side of the range causes thecloud system to dissipate and the warm front to travel asan upper front.

Frontogenesis (the formation of a new front or theregeneration of an old front) may occur in thepressure-trough area that accompanies the front. Thefrontal surface then gradually forms downward as thefrontal system moves away from the mountain andextends to the earth’s surface again (views C and D offig. 4-45). The effect of the mountain range on a warmfront is to widen and prolong the precipitation on thewindward side of the range, while on the leeward sidethe precipitation band is narrowed and weakened, or isnonexistent.

Occluded Fronts

Mountain ranges have much the same effect onoccluded fronts as they do on warm and cold fronts.Cold type occlusions behave as cold fronts, and warmtype occlusions behave as warm fronts. The occlusionprocess is accelerated when a frontal wave approachesa mountain range. The warm front is retarded; but thecold front continues at its normal movement, quicklyovertaking the warm front (views A and B of fig. 4-46).

4-48

A B

C

AG5f0446

Figure 4-46.—Acceleration of the occlusion process and development of a frontal wave cyclone.

When a cold front associated with an occludedfrontal system passes a mountain range, the cold frontmay develop a bulge or wave. In the case of anocclusion, a new and separate low may form at the peakof the warm sector as the occluded front is retarded by amountain range (view C of fig. 4-46). The low developson the peak of the wave because of induced lowpressure that results when air descends on the leewardside of the mountain and warms adiabatically.

The development of a new low on a frontal waveand ultimate separation from its original cyclone is afairly common occurrence. This can occur over openoceans but occurs more frequently along the west coastof mountainous continents and along the west coast ofJapan. The typical stages of this type of frontalmodification are shown in figure 4-47. Orographicfeatures play a great role in certain preferred areas ofthis phenomena, but over the ocean some other factors

must be operative. In some cases, a rapidly movingwave overtakes the slow moving occlusion and may bethe triggering mechanism for this cyclogenesis.

Whatever the exact nature of its causes, this type ofcyclogenesis proceeds with great rapidity. Initially, theold occlusion in view A of figure 4-47 either movesagainst a mountain range or is overtaken by anothercyclone. The occlusion then undergoes frontolysis(view B of fig. 4-47). The new occlusion formsimmediately and soon overshadows its predecessor inboth area and intensity (view C of fig. 4-47). However,the cold occlusion, having greater vertical extension,exerts a certain control on the movement of the newcenter, which at first follows the periphery of the oldcenter. Later, the two centers pivot cyclonically (view Dof fig. 4-47) about a point somewhere on the axisjoining them until the old center has filled and loses itsseparate identity.

4-49

-10

-30

-15

-30

-15

-15

C D

A B

AG5f0447

Figure 4-47.—Stages in the development of a secondary wave cyclone.

This can take place with either a warm or coldocclusion. If it occurs near a west coast in winter, thereis a good chance the new occlusion is warm. Thisformation of a secondary wave cyclone, the dissipationof the original occluded front, and the rapiddevelopment of a new occlusion is sometimes calledskagerraking, pressure jump, or bent-back occlusion.

EFFECTS OF UNDERLYING SURFACES

The migration of a frontal system from one areaand type of underlying surface to another often has agreat modifying effect. It may cause the front to beregenerated in some instances or to dissipate in others.This transition affects cyclones, air masses, and fronts.

Movement Over Land Surfaces

So far, we have established that frontal systemsgenerally weaken when moving from water to landsurfaces. Once these systems are over land, furthermodification can be expected. A front that has justcrossed the mountains and has weakened remains weakor dissipates unless something occurs to strengthen thecontrast between the air masses. If a cold front has justmoved onshore in winter and encounters ice and snowcover over the western half of the United States, themaritime air behind the front quickly takes on coldercontinental properties. The cold underlying surfacemay totally destroy the cold front, especially if theassociated air mass is moving slowly. On the otherhand, if the front is moving quickly enough that it is nottotally destroyed or modified by the colder surface, itmay quickly regenerate as it approaches a warmerunderlying surface and air mass. These normally existover the eastern half of the United States. In thisparticular situation, the air behind the front is muchcolder than when it started. As the front arrives at theedge of the snow field, it probably will encounterwarmer moist air from the gulf or the ocean. Thissituation quickly results in frontogenesis because of asharp air mass contrast. Strong lifting by the wedge ofapproaching cold air results in severe thunderstormsand abundant precipitation along the frontal surface.

If the ice and snow field does not exist over thewestern half of the United States, then the weakenedfront gradually strengthens as it approaches the warmereastern United States. The weather will not be asintense; however, the cold front will have a much widerband of clouds and precipitation. With this situation, airmass contrast is not strong. If the air masses behind andahead of the front are weak, the front becomes

stationary over the extreme southeast United States.The frontal systems are usually oriented in anortheast-southwest direction and occur mostly duringthe summer and autumn months. Frequently, stablewaves develop and travel along this frontal system,causing unfavorable weather conditions. When thesewaves move out to sea and warmer moist air is broughtinto them, they become unstable waves and areregenerated as they move across the ocean.

As the cold fronts cross the AppalachianMountains, they normally weaken once again becausewarm moist air is cut off. After passage over themountains, warm Gulf Stream waters quickly resupplythe frontal surface with the moisture and warm airneeded for the front to strengthen.

Land to Water Migration

Once a cold front moves offshore, most forecastersand analysts forget about them and concentrate on thenext approaching weather. When a front moves into theAtlantic, the weather generally becomes more intense,especially during fall and winter. While your stationmay be relaxing to some degree and enjoying the clearskies after frontal passage, Bermuda and ships at seaare most likely bracing for gale force wind and severethunderstorm activity.

In middle latitudes, ocean currents carry warmwater away from the equator along the eastern coasts ofcontinents and carry cold water toward the equatoralong the western coasts of continents. The most activefrontal zones of the winter season are found where coldcontinental air moves over warm water off easterncoasts. This situation is noticeable off the eastern coastof the United States over the Atlantic Ocean. As a coldfront moves off the coast and over the Gulf Stream, itintensifies, and frequently wave development occursnear the Cape Hatteras area. This gives the eastern coastof the United States much cloudiness and precipitation.This system and its newly intensified front eventuallyreaches Bermuda. A similar situation occurs off theeastern coast of Japan. That area in the Pacificgenerates more cyclones than any other area in theworld.

REVIEW QUESTIONS

Q4-20. What two effects cause the modification offronts?

Q4-21. What normally happens to a cold front thatmoves off the eastern coast of the UnitedStates in the winter?

4-50