CHAPTER 1

FUNDAMENTALS OF METEOROLOGY

Meteorology is the study of atmosphericphenomena. This study consists of physics, chemistry,and dynamics of the atmosphere. It also includes manyof the direct effects the atmosphere has upon Earth’ssurface, the oceans, and life in general. In this manualwe will study the overall fundamentals of meteorology,a thorough description of atmospheric physics andcirculation, air masses, fronts, and meteorologicalelements. This information supplies the necessarybackground for you to understand chart analysis,tropical analysis, satellite analysis, and chartinterpretation.

SYSTEM OF MEASUREMENT

LEARNING OBJECTIVE: Recognize theunits of measure used in the Metric System andthe English System and how these systems ofmeasurement are used in Meteorology.

To work in the field of meteorology, you must havea basic understanding of the science of measurement(metrology). When you can measure what you aretalking about and express it in numerical values, youthen have knowledge of your subject. To measure howfar something is moved, or how heavy it is, or how fastit travels; you may use a specific measurement system.There are many such systems throughout the worldtoday. The Metric System (CGS,centimeter-gram-second) has been recognized for usein science and research. Therefore, that system isdiscussed in the paragraphs that follow, with briefpoints of comparison to the English System (FPS,foot-pound-second). The metric units measure length,weight, and time, respectively. The derivation of thoseunits is described briefly.

LENGTH

To familiarize you with the conventional units ofmetric length, start with the meter. The meter is slightlylarger than the English yard (39.36 inches vs. 36inches). Prefixes are used in conjunction with the meterto denote smaller or larger units of the meter. Eachlarger unit is ten times larger than the next smaller unit.(See table 1-1.).

Table 1-1.—Common Prefixes in the Metric System

Prefix1 SymbolDecimal

ValueScientific Notation

Kilo K 1000 103

Hecto H 100 102

Deka D 10 101

Deci d .1 10-1

Centi c .01 10-2

Milli m .001 10-3

1These prefixes are used with all metric units such asmeters, grams, liters, and seconds (eg., kilometers,hectometers, centiliters, milliseconds).

Since the C in CGS represents centimeters (cm)you should see from table 1-1 that the centimeter isone-hundredth of a meter, .O1M, or 10-2 M.Conversely, 1 M equals 100 cm. To describe a gram,the G in the CGS system, you must first have afamiliarization with area and volume.

AREA AND VOLUME

A square has four equal sides and it is a one-planefigure—like a sheet of paper. To determine how muchsurface area is enclosed within the square you multiplythe length of one side by the length of the other equalside. If the sides were 1 centimeter (cm) in length thearea of the square would be 1 cm × 1 cm = 1 square cm,or 1 cm2. If squares having an area of 1 cm2 werestacked on top of each other until the stack was 1 cmtall, you would end up with a cube whose sides wereeach 1-cm in length. To determine the volume of thecube you simply multiply the length by the width andheight. Because each side is 1 cm you end up with avolume of 1 cubic centimeter (cm3) (1 cm × 1 cm × 1cm = 1 cm3). More simply stated, multiply the area ofone side of the cube by the height of the cube. Once youunderstand how the volume of a cube is determined,you are now ready to review the G in the CGS system.

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WEIGHT

The conventional unit of weight in the metricsystem is the gram (gm). You could use table 1-1 andsubstitute the word gram for meter and the symbol (gm)for the symbol (M). You would then have a table formetric weight. The gram is the weight of 1 cm3 of purewater at 4°C. At this point it may be useful to comparethe weight of an object to its mass. The weight of the 1cm3 of water is 1 gin. Weight and mass are proportionalto each other. However, the weight of the 1 cm3 of waterchanges as you move away from the gravitationalcenter of Earth. In space the 1 cm3 of water isweightless, but it is still a mass. Mass is expressed as afunction of inertia/acceleration, while weight is afunction of gravitational force. When we express themovement of an object we use the terms mass andacceleration.

TIME

Time is measured in hours, minutes, and seconds inboth systems. Hence, the second need not be explainedin the CGS system. With knowledge of how the CGSsystem can be used to express physical entities, younow have all the background to express such things asdensity and force.

DENSITY

With the previous explanation of grams andcentimeters, you should be able to understand howphysical factors can be measured and described. Forexample, density is the weight something has per unitof volume. The density of water is given as 1 gram percubic centimeter or 1 gm/cm. By comparison, thedensity of water in the English system is 62.4 poundsper cubic foot or 62.4 lb/ft3.

FORCE

Force is measured in dynes. A dyne is the force thatmoves a mass of 1 gram, 1 centimeter per squaresecond. This is commonly written as gin cm per sec2,gin cm/sec/sec or gm/cm/sec2. The force necessary fora gram to be accelerated at 980.665 cm/sec2 at 45°latitude is 980.665 dynes. For more detailed conversionfactors commonly used in meteorology andoceanography, refer to Smithsonian MeteorologyTables.

REVIEW QUESTIONS

Q1-1. What units does the metric (CGS) systemmeasure?

Q1-2. What is the difference between weight andmass?

Q1-3. What does a dyne measure?

EARTH-SUN RELATIONSHIP

LEARNING OBJECTIVE: Describe howradiation and insolation are affected by theEarth-Sun relationship.

The Sun is a great thermonuclear reactor about 93million miles from Earth. It is the original source ofenergy for the atmosphere and life itself. The Sun’senergy is efficiently stored on Earth in such things asoil, coal, and wood. Each of these was produced bysome biological means when the Sun acted upon livingorganisms. Our existence depends on the Sun becausewithout the Sun there would be no warmth on Earth, noplants to feed animal life, and no animal life to feedman.

The Sun is important in meteorology because allnatural phenomena can be traced, directly or indirectly,to the energy received from the Sun. Although the Sunradiates its energy in all directions, only a small portionreaches our atmosphere. This relatively small portion ofthe Sun’s total energy represents a large portion of theheat energy for our Earth. It is of such importance inmeteorology that every Aerographer’s Mate shouldhave at least a basic knowledge about the Sun and theeffects it has on Earth’s weather.

SUN

The Sun may be regarded as the only source of heatenergy that is supplied to earth’s surface and theatmosphere. All weather and motions in the atmosphereare due to the energy radiated from the Sun.

The Sun’s core has a temperature of 15,000,000°Kand a surface temperature of about 6,000°K (10,300°F).The Sun radiates electromagnetic energy in alldirections. However, Earth intercepts only a smallfraction of this energy. Most of the electromagneticenergy radiated by the Sun is in the form of light waves.Only a tiny fraction is in the form of heat waves. Evenso, better than 99.9 percent of Earth’s heat is derivedfrom the Sun in the form of radiant energy.

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Solar Composition

The Sun may be described as a globe of gas heatedto incandescence by thermonuclear reactions fromwithin the central core.

The main body of the Sun, although composed ofgases, is opaque and has several distinct layers. (See fig.1-1.) The first of these layers beyond the radiative zoneis the convective zone. This zone extends very nearly tothe Sun’s surface. Here, heated gases are raisedbuoyantly upwards with some cooling occurring andsubsequent convective action similar to that, whichoccurs within Earth’s atmosphere. The next layer is awell-defined visible surface layer referred to as thephotosphere. The bottom of the photosphere is the solarsurface. In this layer the temperature has cooled to asurface temperature of 6,000°K at the bottom to4,300°K at the top of the layer. All the light and heat of

the Sun is radiated from the photosphere. Above thephotosphere is a more transparent gaseous layerreferred to as the chromosphere with a thickness ofabout 1,800 miles (3,000 km). It is hotter than thephotosphere. Above the chromosphere is the corona, alow-density high temperature region. It is extended farout into interplanetary space by the solar wind—asteady outward streaming of the coronal material.Much of the electromagnetic radiation emissionsconsisting of gamma rays through x-rays, ultraviolet,visible and radio waves, originate in the corona.

Within the solar atmosphere we see the occurrenceof transient phenomena (referred to as solar activity),just as cyclones, frontal systems, and thunderstormsoccur within the atmosphere of Earth. This solaractivity may consist of the phenomena discussed in thefollowing paragraphs that collectively describe thefeatures of the solar disk (the visual image of the outer

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CORONA

CHRO

MO

SPHER

E

PHOTOSPHERE

RADIATIVE ZONE

SOLARATMOSPHERE

SEVERALSOLARDIAMETERSIN DEPTH 3,000Km

SOLAR SURFACETEMPERATUREAPPROX. 6,000 K

O

CENTRAL CORE(THERMONUCLEARREACTIONS) APPROX.15,000,000 KO

CO

NVEC

TIVE

ZONE

AGf0101

Figure 1-1.—One-quarter cross-section depicting the solar structure.

surface of the sun as observed from outside regions).(See fig. 1-2).

Solar Prominences/Filaments

Solar prominences/filaments are injections of gasesfrom the chromosphere into the corona. They appear asgreat clouds of gas, sometimes resting on the Sun’ssurface and at other times floating free with no visibleconnection. When viewed against the solar disk, theyappear as long dark ribbons and are called filaments.When viewed against the solar limb (the dark outeredge of the solar disk), they appear bright and are calledprominences. (See fig. 1-2.) They display a variety ofshapes, sizes, and activity that defy general description.

They have a fibrous structure and appear to resist solargravity. They may extend 18,500 to 125,000 miles(30,000 to 200,000 km) above the chromosphere. Themore active types have temperatures of 10,000°K ormore and appear hotter than the surroundingatmosphere.

Sunspots

Sunspots are regions of strong localized magneticfields and indicate relatively cool areas in thephotosphere. They appear darker than theirsurroundings and may appear singly or in morecomplicated groups dominated by larger spots near thecenter. (See fig. 1-2).

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SOLAR PROMINENCES

PLAGE

FLARE

SUNSPOTS

(FILAMENTS)

Figure 1-2.—Features of the solar disk.

Sunspots begin as small dark areas known as pores.These pores develop into full-fledged spots in a fewdays, with maximum development occurring in about 1to 2 weeks. When sunspots decay the spot shrinks insize and its magnetic field also decreases in size. Thislife cycle may consist of a few days for small spots tonear 100 days for larger groups. The larger spotsnormally measure about 94,500 miles (120,000 kin)across. Sunspots appear to have cyclic variations inintensity, varying through a period of about 8 to 17years. Variation in number and size occurs throughoutthe sunspot cycle. As a cycle commences, a few spotsare observed at high latitudes of both solarhemispheres, and the spots increase in size and number.They gradually drift equatorward as the cycleprogresses, and the intensity of the spots reach amaximum in about 4 years. After this period, decay setsin and near the end of the cycle only a few spots are leftin the lower latitudes (5° to 10°).

Plages

Plages are large irregular bright patches thatsurround sunspot groups. (See fig. 1-2). They normallyappear in conjunction with solar prominences orfilaments and may be systematically arranged in radialor spiral patterns. Plages are features of the lowerchromosphere and often completely or partiallyobscure an underlying sunspot.

Flares

Solar flares are perhaps the most spectacular of theeruptive features associated with solar activity. (See fig.1-2). They look like flecks of light that suddenly appearnear activity centers and come on instantaneously asthough a switch were thrown. They rise sharply to peakbrightness in a few minutes, then decline moregradually. The number of flares may increase rapidlyover an area of activity. Small flare-like brighteningsare always in progress during the more active phase ofactivity centers. In some instances flares may take theform of prominences, violently ejecting material intothe solar atmosphere and breaking into smallerhigh-speed blobs or clots. Flare activity appears to varywidely between solar activity centers. The greatest flareproductivity seems to be during the week or 10 dayswhen sunspot activity is at its maximum.

Flares are classified according to size andbrightness. In general, the higher the importanceclassification, the stronger the geophysical effects.Some phenomena associated with solar flares haveimmediate effects; others have delayed effects (15minutes to 72 hours after flare).

Solar flare activity produces significant disruptionsand phenomena within Earth’s atmosphere. Duringsolar flare activity, solar particle streams (solar winds)are emitted and often intercept Earth. These solarparticles are composed of electromagnetic radiation,which interacts with Earth’s ionosphere. This results inseveral reactions such as: increased ionization(electrically charging neutral particles), photo chemicalchanges (absorption of radiation), atmospheric heating,electrically charged particle motions, and an influx ofradiation in a variety of wavelengths and frequencieswhich include radio and radar frequencies.

Some of the resulting phenomena include thedisruption of radio communications and radardetection. This is due to ionization, incoming radiowaves, and the motion of charged particles. Satelliteorbits can be affected by the atmospheric heating andsatellite transmissions may be affected by all of thereactions previously mentioned. Geomagneticdisturbances like the aurora borealis and auroraAustralia result primarily from the motion ofelectrically charged particles within the ionosphere.

EARTH

Of the nine planets in our solar system, Earth is thethird nearest to (or from) the Sun. Earth varies indistance from the Sun during the year. The Sun is 94million miles (150,400,000 km) in summer and 91million miles (145,600,000 km) in winter.

Motions

Earth is subject to four motions in its movementthrough space: rotation about its axis, revolution aroundthe Sun, processional motion (a slow conical movementor wobble) of the axis, and the solar motion (themovement of the whole solar system with space). Ofthe four motions affecting Earth, only two are of anyimportance to meteorology.

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The first motion is rotation. Earth rotates on its axisonce every 24 hours. One-half of the Earth’s surface istherefore facing the Sun at all times. Rotation aboutEarth’s axis takes place in an eastward direction. Thus,the Sun appears to rise in the east and set in the west.(See fig. 1-3.)

The second motion of Earth is its revolution aroundthe Sun. The revolution around the Sun and the tilt ofEarth on its axis are responsible for our seasons. Earthmakes one complete revolution around the Sun inapproximately 365 1/4 days. Earth’s axis is at an angleof 23 1/2° to its plane of rotation and points in a nearlyfixed direction in space toward the North Star (Polaris).

Solstices and Equinoxes

When Earth is in its summer solstice, as shown forJune in figure 1-4, the Northern Hemisphere is inclined23 1/2° toward the Sun. This inclination results in moreof the Sun’s rays reaching the Northern Hemispherethan the Southern Hemisphere. On or about June 21,

direct sunlight covers the area from the North Poledown to latitude 66 1/2°N (the Arctic Circle). The areabetween the Arctic Circle and the North Pole isreceiving the Sun’s rays for 24 hours each day. Duringthis time the most perpendicular rays of the Sun arereceived at 23 l/2°N latitude (the Tropic Of Cancer).Because the Southern Hemisphere is tilted away fromthe Sun at this time, the indirect rays of the Sun reachonly to 66 1/2°S latitude (the Antarctic Circle).Therefore, the area between the Antarctic Circle andthe South Pole is in complete darkness. Note carefullythe shaded and the not shaded area of Earth in figure 1-4for all four positions.

At the time of the equinox in March and again inSeptember, the tilt of Earth’s axis is neither toward noraway from the Sun. For these reasons Earth receives anequal amount of the Sun’s energy in both the NorthernHemisphere and the Southern Hemisphere. During thistime the Sun’s rays shine most perpendicularly at theequator.

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MIDNIGHTSUNSET

SUNRISENOON

Figure 1-3.—Rotation of the Earth about its axis (during equinoxes).

In December, the situation is exactly reversed fromthat in June. The Southern Hemisphere now receivesmore of the Sun’s direct rays. The most perpendicularrays of the Sun are received at 23 1/2°S latitude (theTropic Of Capricorn). The southern polar region is nowcompletely in sunshine and the northern polar region iscompletely in darkness.

Since the revolution of Earth around the Sun is agradual process, the changes in the area receiving theSun’s rays and the changes in seasons are gradual.However, it is customary and convenient to mark thesechanges by specific dates and to identify them byspecific names. These dates are as follows:

1. March 21. The vernal equinox, when Earth’saxis is perpendicular to the Sun’s rays. Spring begins inthe Northern Hemisphere and fall begins in theSouthern Hemisphere.

2. June 21. The summer solstice, when Earth’saxis is inclined 23 1/2° toward the Sun and the Sun hasreached its northernmost zenith at the Tropic of Cancer.Summer officially commences in the NorthernHemisphere; winter begins in the SouthernHemisphere.

3. September 22. The autumnal equinox, whenEarth’s axis is again perpendicular to the Sun’s rays.This date marks the beginning of fall in the NorthernHemisphere and spring in the Southern Hemisphere. Itis also the date, along with March 21, when the Sunreaches its highest position (zenith) directly over theequator.

4. December 22. The winter solstice, when theSun has reached its southernmost zenith position at theTropic of Capricorn. It marks the beginning of winter inthe Northern Hemisphere and the beginning of summerin the Southern Hemisphere.

In some years, the actual dates of the solstices andthe equinoxes vary by a day from the dates given here.This is because the period of revolution is 365 1/4 daysand the calendar year is 365 days except for leap yearwhen it is 366 days.

Because of its 23 1/2° tilt and its revolution aroundthe Sun, five natural light (or heat) zones according tothe zone's relative position to the Sun's rays mark Earth.Since the Sun is ALWAYS at its zenith between theTropic of Cancer and the Tropic of Capricorn, this is thehottest zone. It is called the Equatorial Zone, the TorridZone, the Tropical Zone, or simply the Tropics.

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MARCH 21

SEPTEMBER22

JUNE 21

SUN

N/P

S/P

N/P

S/P

N/P

S/P

DECEMBER22

N/P

S/P

Figure 1-4.—Revolution of Earth around the sun.

The zones between the Tropic of Cancer and theArctic Circle and between the Tropic of Capricorn andthe Antarctic Circle are the Temperate Zones. Thesezones receive sunshine all year, but less of it in theirrespective winters and more of it in their respectivesummers.

The zones between the Arctic Circle and the NorthPole and between the Antarctic Circle and the SouthPole receive the Sun’s rays only for parts of the year.(Directly at the poles there are 6 months of darknessand 6 months of sunshine.) This, naturally, makes themthe coldest zones. They are therefore known as theFrigid or Polar Zones.

RADIATION

The term "radiation" refers to the process by whichelectromagnetic energy is propagated through space.Radiation moves at the speed of light, which is 186,000miles per second (297,600 km per second) and travelsin straight lines in a vacuum. All of the heat received byEarth is through this process. It is the most importantmeans of heat transfer.

Solar radiation is defined as the totalelectromagnetic energy emitted by the Sun. The Sun’s

surface emits gamma rays, x-rays, ultraviolet, visiblelight, infrared, heat, and electromagnetic waves.Although the Sun radiates in all wavelengths, abouthalf of the radiation is visible light with most of theremainder being infrared. (See figure 1-5.)

Energy radiates from a body by wavelengths,which vary inversely with the temperature of that body.Therefore, the Sun, with an extremely hot surfacetemperature, emits short wave radiation. Earth has amuch cooler temperature (15°C average) and thereforereradiates the Sun’s energy or heat with long waveradiation.

INSOLATION

Insolation (an acronym for INcoming SOLarradiATION) is the rate at which solar radiation isreceived by a unit horizontal surface at any point on orabove the surface of Earth. In this manual, insolation isused when speaking about incoming solar radiation.

There are a wide variety of differences in theamounts of radiation received over the various portionsof Earth’s surface. These differences in heating areimportant and must be measured or otherwisecalculated to determine their effect on the weather.

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SCHEMATIC DIAGRAM 0F THE DISTRIBUTION OF ENERGY IN THE SOLAR SPECTRUM.(NOT TO SCALE). THE NUMBERS ARE PERCENTAGES OF THE SOLAR CONSTANT . THEFIGURE FOR THE RADIO ENERGY IS FOR THE OBSERVED BAND FROM 15 TO 30,000 MHZ.

EXTREME U.V.AND RAYS

NEARU.V.

VISIBLE INFRARED RADIO

41

527

10 -3

10 10 10 10 10 10 10 10 10 10 10-6 -4 -2 2 4 6 8 10 12 14 16

10 -10

WAVELENGTHS IN MILLIMICRONS

VISIBLE SPECTRUM

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COSMICRAYS

GAMMARAYS

X-RAYS ULTRA-VIOLETRAYS

INFRA-REDRAYS

HERTZIANWAVES

RADIOWAVES

LONGELECTRICAL

OSCILLATIONS

400mu 700mu

1

Figure 1-5.—Electromagnetic spectrum.

The insolation received at the surface of Earthdepends upon the solar constant (the rate at which solarradiation is received outside Earth’s atmosphere), thedistance from the Sun, inclination of the Sun’s rays, andthe amount of insolation depleted while passingthrough the atmosphere. The last two are the importantvariable factors.

Depletion of Solar Radiation

If the Sun’s radiation was not filtered or depleted insome manner, our planet would soon be too hot for lifeto exist. We must now consider how the Sun’s heatenergy is both dispersed and depleted. This isaccomplished through dispersion, scattering,reflection, and absorption.

DISPERSION.—Earlier it was learned thatEarth’s axis is inclined at an angle of 23 1/2°. Thisinclination causes the Sun’s rays to be received on thesurface of Earth at varying angles of incidence,depending on the position of Earth. When the Sun’srays are not perpendicular to the surface of Earth, theenergy becomes dispersed or spread out over a greaterarea (figure.1-6). If the available energy reaching theatmosphere is constant and is dispersed over a greaterarea, the amount of energy at any given point within thearea decreases, and therefore the temperature is lower.Dispersion of insolation in the atmosphere is caused bythe rotation of Earth. Dispersion of insolation also takesplace with the seasons in all latitudes, but especially inthe latitudes of the polar areas.

SCATTERING.—About 25 percent of theincoming solar radiation is scattered or diffused by theatmosphere. Scattering is a phenomenon that occurswhen solar radiation passes through the air and some ofthe wavelengths are deflected in all directions bymolecules of gases, suspended particles, and water

vapor. These suspended particles then act like a prismand produce a variety of colors. Various wavelengthsand particle sizes result in complex scattering affectsthat produce the blue sky. Scattering is also responsiblefor the red Sun at sunset, varying cloud colors at sunriseand sunset, and a variety of optical phenomena.

Scattering always occurs in the atmosphere, butdoes not always produce dramatic settings. Undercertain radiation wavelength and particle sizeconditions all that can be seen are white clouds and awhitish haze. This occurs when there is a high moisturecontent (large particle size) in the air and is calleddiffuse reflection. About two-thirds of the normallyscattered radiation reaches earth as diffuse skyradiation. Diffuse sky radiation may account for almost100 percent of the radiation received by polar stationsduring winter.

REFLECTION.—Reflection is the processwhereby a surface turns a portion of the incident backinto the medium through which the radiation came.

A substance reflects some insolation. This meansthat the electromagnetic waves simply bounce backinto space. Earth reflects an average of 36 percent of theinsolation. The percent of reflectivity of allwavelengths on a surface is known as its albedo. Earth’saverage albedo is from 36 to 43 percent. That is, Earthreflects 36 to 43 percent of insolation back into space.In calculating the albedo of Earth, the assumption ismade that the average cloudiness over Earth is 52percent. All surfaces do not have the same degree ofreflectivity; consequently, they do not have the samealbedo. Some examples are as follows:

1. Upper surfaces of clouds reflect from 40 to 80percent, with an average of about 55 percent.

2. Snow surfaces reflect over 80 percent ofincoming sunlight for cold, fresh snow and as lowas 50 percent for old, dirty snow.

3. Land surfaces reflect from 5 percent ofincoming sunlight for dark forests to 30 percent fordry land.

4. Water surfaces (smooth) reflect from 2 percent,when the Sun is directly overhead, to 100 percentwhen, the Sun is very low on the horizon. Thisincrease is not linear. When the Sun is more than25°above the horizon, the albedo is less than 10percent. In general, the albedo of water is quite low.

When Earth as a whole is considered, clouds aremost important in determining albedo.

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SUN

OBLIQUE RAYSDISPERSED OVERA LARGER AREATHANPERPENDICULARRAYS

S

N

EQUATOR

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Figure 1-6.—Dispersion of insolation.

ABSORPTION.—Earth and its atmosphereabsorb about 64 percent of the insolation. Land andwater surfaces of Earth absorb 51 percent of thisinsolation. Ozone, carbon dioxide, and water vapordirectly absorb the remaining 13 percent. These gasesabsorb the insolation at certain wavelengths. Forexample, ozone absorbs only a small percentage of theinsolation. The portion or type the ozone does absorb iscritical since it reduces ultraviolet radiation to a levelwhere animal life can safely exist. The most importantabsorption occurs with carbon dioxide and water vapor,which absorb strongly over a broader wavelength band.Clouds are by far the most important absorbers ofradiation at essentially all wavelengths. In sunlightclouds reflect a high percentage of the incident solarradiation and account for most of the brightness ofEarth as seen from space.

There are regions, such as areas of clear skies,where carbon dioxide and water vapor are at aminimum and so is absorption. These areas are calledatmospheric windows and allow insolation to passthrough the atmosphere relatively unimpeded.

Greenhouse Effect

The atmosphere conserves the heat energy of Earthbecause it absorbs radiation selectively. Most of thesolar radiation in clear skies is transmitted to Earth’ssurface, but a large part of the outgoing terrestrialradiation is absorbed and reradiated back to the surface.This is called the greenhouse effect. A greenhousepermits most of the short-wave solar radiation to passthrough the glass roof and sides, and to be absorbed bythe floor, ground or plants inside. These objectsreradiate energy at their temperatures of about 300°K,which is a higher temperature than the energy that wasinitially received. The glass absorbs the energy at thesewavelengths and sends part of it back into thegreenhouse, causing the inside of the structure tobecome warmer than the outside. The atmosphere actssimilarly, transmitting and absorbing in somewhat thesame way as the glass. If the greenhouse effect did notexist, Earth’s temperature would be 35°C cooler thanthe 15°C average temperature we now enjoy, becausethe insolation would be reradiated back to space.

Of course, the atmosphere is not a contained spacelike a greenhouse because there are heat transportmechanisms such as winds, vertical currents, andmixing with surrounding and adjacent cooler air.

RADIATION (HEAT) BALANCE IN THEATMOSPHERE

The Sun radiates energy to Earth, Earth radiatesenergy back to space, and the atmosphere radiatesenergy also. As is shown in figure 1-7, a balance ismaintained between incoming and outgoing radiation.This section of the lesson explains the various radiationprocesses involved in maintaining this critical balanceand the effects produced in the atmosphere.

We have learned that an object reradiates energy ata higher temperature. Therefore, the more the Sun heatsEarth, the greater the amount of heat energy Earthreradiates. If this rate of heat loss/gain did not balance,Earth would become continuously colder or warmer.

Terrestrial (Earth) Radiation

Radiation emitted by Earth is almost entirelylong-wave radiation. Most of the terrestrial radiation isabsorbed by the water vapor in the atmosphere andsome by other gases (about 8 percent is radiateddirectly to outer space). This radiant energy isreradiated in the atmosphere horizontally andvertically. Horizontal flux (flow or transport) of energyneed not be considered due to a lack of horizontaltemperature differences. The vertical, upward ordownward, flux is of extreme significance.

Convection and turbulence carry aloft some of thisradiation. Water vapor, undergoing thecondensation-precipitation-evaporation cycle(hydrological cycle), carries the remainder into theatmosphere.

Atmospheric Radiation

The atmosphere reradiates to outer space most ofthe terrestrial radiation (about 43 percent) andinsolation (about 13 percent) that it has absorbed. Someof this reradiation is emitted earthward and is known ascounterradiation. This radiation is of great importancein the greenhouse effect.

Heat Balance and Transfer in the Atmosphere

Earth does not receive equal radiation at all pointsas was shown in figure 1-4. The east-west rotation ofEarth provides equal exposure to sunlight but latitudeand dispersion do affect the amount of incidentradiation received. The poles receive far less incidentradiation than the equator. This uneven heating is calleddifferential insolation.

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Due to this differential insolation the tropicalatmosphere is constantly being supplied heat and thetemperature of the air is thus higher than in areaspoleward. Because of the expansion of warm air, thiscolumn of air is much thicker and lighter than over thepoles. At the poles Earth receives little insolation andthe column or air is less thick and heavier. Thisdifferential in insolation sets up a circulation thattransports warm air from the Tropics poleward aloft andcold air from the poles equatorward on the surface. (Seefig. 1-8.) Modifications to this general circulation arediscussed in detail later in this training manual.

This is the account of the total radiation. Some ofthe radiation makes several trips, being absorbed,reflected, or reradiated by Earth or the atmosphere.Insolation comes into the atmosphere and all of it isreradiated. How many trips it makes while in ouratmosphere does not matter. The direct absorption ofradiation by Earth and the atmosphere and thereradiation into space balance. If the balance did notexist, Earth and its atmosphere, over a period of time,would steadily gain or lose heat.

Although radiation is considered the mostimportant means of heat transfer, it is not the onlymethod. There are others such as conduction,convection, and advection that also play an importantpart in meteorological processes.

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OUTER SPACE

UPPER ATMOSPHERE

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NOTE:36% OF INCOMING INSULATIONINITIALLY REFLECTED

RADIATED DIRECTLYTO OUTER SPACE8%

RERADIATEDBY ATMOSPHERETO OUTER SPACE56%

INCOMING

SHORT WAVE

RADIATION

REFLECTED BY

EARTH, CLOUDS

AND ATMOSPHERE

TO OUTER SPACE

36%

51%ABSORBEDBY EARTH

13%ABSORBED BYATMOSPHERE

OUTGOINGLONG WAVERADIATION

HYDROLOGICCYCLE

Figure 1-7.—Radiation balance in the atmosphere.

NORTHPOLE

SOUTHPOLE

EQUATOR AREA OFGREATEST

INSOLATION

AREA OF LEASTINSOLATION

AREA OF LEASTINSOLATION

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Figure 1-8.—Beginning of a circulation.

REVIEW QUESTIONS

Q1-4. What are sunspots?

Q1-5. In the Southern Hemisphere, approximatelywhat date will the greatest amount ofincoming solar radiation be received?

Q1-6. What percent of the earth's insolation do landand water absorb?

Q1-7.What is the effect on a polar air column inrelation to a column of air over the equator?

PRESSURE

LEARNING OBJECTIVE: Describe howpressure is measured and determine how theatmosphere is affected by pressure.

DEFINITION AND FORMULA

Pressure is the force per unit area. Atmosphericpressure is the force per unit area exerted by theatmosphere in any part of the atmospheric envelope.Therefore, the greater the force exerted by the air forany given area, the greater the pressure. Although thepressure varies on a horizontal plane from day to day,the greatest pressure variations are with changes inaltitude. Nevertheless, horizontal variations of pressureare ultimately important in meteorology because thevariations affect weather conditions.

Pressure is one of the most important parameters inmeteorology. Knowledge of the distribution of air andthe resultant variations in air pressure over the earth isvital in understanding Earth’s fascinating weatherpatterns.

Pressure is force, and force is related toacceleration and mass by Newton’s second law. Thislaw states that acceleration of a body is directlyproportional to the force exerted on the body andinversely proportional to the mass of that body. It maybe expressed as

aF

mor F ma= =

“A” is the acceleration, “F” is the force exerted, and"in" is the mass of the body. This is probably the mostimportant equation in the mechanics of physics dealingwith force and motion.

NOTE: Be sure to use units of mass and not units ofweight when applying this equation.

STANDARDS OF MEASUREMENT

Atmospheric pressure is normally measured inmeteorology by the use of a mercurial or aneroidbarometer. Pressure is measured in many differentunits. One atmosphere of pressure is 29.92 inches ofmercury or 1,013.25 millibars. These measurementsare made under established standard conditions.

STANDARD ATMOSPHERE

The establishment of a standard atmosphere wasnecessary to give scientists a yardstick to measure orcompare actual pressure with a known standard. In theInternational Civil Aeronautical Organization (ICAO),the standard atmosphere assumes a mean sea leveltemperature of 59°F or 15°C and a standard sea levelpressure of 1,013.25 millibars or 29.92 inches ofmercury. It also has a temperature lapse rate (decrease)of 3.6°F per 1000 feet or 0.65°C per 100 meters up to 11kilometers and a tropopause and stratospheretemperature of -56.5°C or -69.7°F.

VERTICAL DISTRIBUTION

Pressure at any point in a column of water, mercury,or any fluid, depends upon the weight of the columnabove that point. Air pressure at any given altitudewithin the atmosphere is determined by the weight ofthe atmosphere pressing down from above. Therefore,the pressure decreases with altitude because the weightof the atmosphere decreases.

It has been found that the pressure decreases by halffor each 18,000-foot (5,400-meter) increase in altitude.Thus, at 5,400 meters one can expect an averagepressure of about 500 millibars and at 36,000 feet(10,800 meters) a pressure of only 250 millibars, etc.Therefore, it may be concluded that atmosphericpressures are greatest at lower elevations because thetotal weight of the atmosphere is greatest at thesepoints.

There is a change of pressure whenever either themass of the atmosphere or the accelerations of themolecules within the atmosphere are changed.Although altitude exerts the dominant control,temperature and moisture alter pressure at any givenaltitude—especially near Earth’s surface where heatand humidity, are most abundant. The pressurevariations produced by heat and humidity with heatbeing the dominant force are responsible for Earth’swinds through the flow of atmospheric mass from anarea of higher pressure to an area of lower pressure.

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PASCAL’S LAW

Pascal's Law is an important law in atmosphericphysics. The law states that fluids (including gases suchas Earth’s atmosphere) transmit pressure in alldirections. Therefore, the pressure of the atmosphere isexerted not only downward on the surface of an object,but also in all directions against a surface that isexposed to the atmosphere.

REVIEW QUESTIONS

Q1-8. What is the definition of pressure?

Q1-9. With a sea level pressure reading of 1000 mb,what would be the approximate pressure at18,000 feet?

Q1-10. What environmental changes have the biggesteffect on pressure changes?

TEMPERATURE

LEARNING OBJECTIVE: Describe howtemperature is measured and determine howthe atmosphere is affected by temperature.

DEFINITION

Temperature is the measure of molecular motion.Its intensity is determined from absolute zero (Kelvinscale), the point which all molecular motion stops.Temperature is the degree of hotness or coldness, or itmay be considered as a measure of heat intensity.

TEMPERATURE SCALES

Long ago it was recognized that uniformity in themeasurement of temperature was essential. It would beunwise to rely on such subjective judgments oftemperature as cool, cooler, and coolest; therefore,arbitrary scales were devised. Some of them aredescribed in this section. They are Fahrenheit, Celsius,and absolute (Kelvin) scales. These are the scales usedby the meteorological services of all the countries in theworld. Table 1-2 shows a temperature conversion scalefor Celsius, Fahrenheit, and Kelvin.

Fahrenheit Scale

Gabriel Daniel Fahrenheit invented the Fahrenheitscale about 1710. He was the first to use mercury in athermometer. The Fahrenheit scale has 180 divisions or

degrees between the freezing (32°F) and boiling(212°F) points of water.

Celsius Scale

Anders Celsius devised the Celsius scale during the18th century. This scale has reference points withrespect to water of 0°C for freezing and 100°C forboiling. It should be noted that many publications stillrefer to the centigrade temperature scale. Centigradesimply means graduated in 100 increments, and hasrecently and officially adopted the name of itsdiscoverer, Celsius.

Absolute Scale (Kelvin)

Another scale in wide use by scientists in manyfields is the absolute scale or Kelvin scale, developedby Lord Kelvin of England. On this scale the freezingpoint of water is 273°K and the boiling point of water is373°K. The absolute zero value is considered to be apoint at which theoretically no molecular activityexists. This places the absolute zero at a minus 2730 onthe Celsius scale, since the degree divisions are equal insize on both scales. The absolute zero value on theFahrenheit scale falls at minus 459.6°F.

Scale Conversions

Two scales, Fahrenheit and Celsius, are commonlyused. With the Celsius and Fahrenheit scales, it is oftennecessary to change the temperature value of one scaleto that of the other. Generally a temperature conversiontable, like table 1-2, is used or a temperature computer.If these are not available, you must then use one of thefollowing mathematical methods to convert one scale toanother.

Mathematical Methods

It is important to note that there are 100 divisionsbetween the freezing and boiling points of water on theCelsius scale. There are 180 divisions between thesame references on the Fahrenheit scale. Therefore, onedegree on the Celsius scale equals nine-fifths degree onthe Fahrenheit scale. In converting Fahrenheit values toCelsius values the formula is:

C (F 32 )5

9= − °

In converting Celsius values to Fahrenheit values theformula is:

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F9

5C 32= + °

One way to remember when to use 9/5 and when touse 5/9 is to keep in mind that the Fahrenheit scale hasmore divisions than the Celsius scale. In going fromCelsius to Fahrenheit, multiply by the ratio that is

larger; in going from Fahrenheit to Celsius, use thesmaller ratio.

Another method of converting temperatures fromone scale to another is the decimal method. Thismethod uses the ratio 1°C equals 1.8°F. To findFahrenheit from Celsius, multiply the Celsius value by1.8 and add 32. To find Celsius from Fahrenheit,

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1Fahrenheit temperatures are rounded to the nearest 0.5 degree. For a more exact conversion, utilize thepsychrometric computer or the mathematical method.

Table1-2

Table 1-2.—Temperature Conversion Scale

subtract 32 from the Fahrenheit and divide theremainder by 1.8.

Examples:

1. F = l.8C + 32

Given: 24°C. Find: °F24 × 1.8 = 43.243.2 + 32 = 75.2 or 75°F.

2. C =F 32

1.8

−

Given: 96°F. Find: °C.96 – 32 = 6464 + 1.8 = 35.5 or 36°C

To change a Celsius reading to an absolute value,add the Celsius reading to 273° algebraically. For

example, to find the absolute value of -35°C, you wouldadd minus 35° to 273°K algebraically. That is, you take273° and combine 35° so you use the minus (-) functionto arrive at 238°K.

To change a Fahrenheit reading to an absolutevalue, first convert the Fahrenheit reading to itsequivalent Celsius value. Add this value algebraicallyto 273°. Consequently, 50°F is equivalent to 2830absolute, arrived at by converting 50°F to 10°C andthen adding the Celsius value algebraically to 273°.

VERTICAL DISTRIBUTION

Earth’s atmosphere is divided into layers or zonesaccording to various distinguishing features. (See fig.1-9). The temperatures shown here are generally basedon the latest “U.S. Extension to the ICAO Standard

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AURORA

TEMPERATURE(KINETIC)

PRESSURE(MB)

NOCTILUCENTCLOUDS

0 500 1000 1500 2000

600

500

400

300

200

100

700

2500

10

10

10

10

10

STANDARDTEMPERATURE

TROPOPAUSE

NACREOUSCLOUDS

MT EVEREST180 190 200 210 220 230 240 250 260 270 280 290 300 DEGREES K

DEGREES C

DEGREES F

100

200

300

500

700850

1000

50

50

25

10100

150

200

10-1

1

250

10-2

-3

300

GEOMETRICHEIGHT

F REGION

E REGION

D REGION

10

KILOMETERS

100

90

60

80

70

40

50

10

20

30

STRATOSPHERE

STRATOPAUSE

MESOPAUSE

THERMOSHPERE

MESOSPHERE

TROPOSPHERE

GEOMETRIC HEIGHT

KILOMETERS

(400) MILES

(300)

(200)

(100)

-4

-5

-6

-7

-8

(MILES)(60)

PRESSURE(MB)

TEMPERATURE

(10)

(20)

(30)

(40)

(50)

THOUSOF FEET

-90

-140 -120 -100

-80 -70

-80

-60 -50

-60

-40

-40

-30

-20

-20

0

-10

20

0

32 40

10

60

20

80

30

OZONOSPHERE

E REGION

D REGION

DEG. K

IONOSPH

ERE

F REGION2

F REGION1

AGf0109

Figure 1-9.—Earth's Atmosphere.

Atmosphere” and are representative of mid-latitudeconditions. The extension shown in the insert isspeculative. These divisions are for reference ofthermal structure (lapse rates) or other significantfeatures and are not intended to imply that these layersor zones are independent domains. Earth is surroundedby one atmosphere, not by a number ofsub-atmospheres.

The layers and zones are discussed under twoseparate classifications. One is theMETEOROLOGICAL classification that defines zonesaccording to their significance for the weather. Theother is the ELECTRICAL classification that defineszones according to electrical characteristics of gases ofthe atmosphere.

Meteorological Classification

In the meteorological classification (commencingwith Earth’s surface and proceeding upward) we havethe troposphere, tropopause, stratosphere, stratopause,mesosphere, mesopause, thermosphere, and theexosphere. These classifications are based ontemperature characteristics. (See fig. 1-9 for someexamples.)

TROPOSPHERE.—The troposphere is the layerof air enveloping Earth immediately above Earth’ssurface. It is approximately 5 1/2 miles (29,000 ft or 9kin) thick over the poles, about 7 1/2 miles (40,000 ft or12.5 kin) thick in the mid-latitudes, and about 11 1/2miles (61,000 ft or 19 kin) thick over the Equator. Thefigures for thickness are average figures; they changesomewhat from day to day and from season to season.The troposphere is thicker in summer than in winter andis thicker during the day than during the night. Almostall weather occurs in the troposphere. However, somephenomena such as turbulence, cloudiness (caused byice crystals), and the occasional severe thunderstormtop occur within the tropopause or stratosphere.

The troposphere is composed of a mixture ofseveral different gases. By volume, the composition ofdry air in the troposphere is as follows: 78 percentnitrogen, 21 percent oxygen, nearly 1-percent argon,and about 0.03 percent carbon dioxide. In addition, itcontains minute traces of other gases, such as helium,hydrogen, neon, krypton, and others.

The air in the troposphere also contains a variableamount of water vapor. The maximum amount of watervapor that the air can hold depends on the temperatureof the air and the pressure. The higher the temperature,the more water vapor it can hold at a given pressure.

The air also contains variable amounts ofimpurities, such as dust, salt particles, soot, andchemicals. These impurities in the air are importantbecause of their effect on visibility and the part theyplay in the condensation of water vapor. If the air wereabsolutely pure, there would be little condensation.These minute particles act as nuclei for thecondensation of water vapor. Nuclei, which have anaffinity for water vapor, are called HYGROSCOPICNUCLEI.

The temperature in the troposphere usuallydecreases with height, but there may be inversions forrelatively thin layers at any level.

TROPOPAUSE.—The tropopause is a transitionlayer between the troposphere and the stratosphere. It isnot uniformly thick, and it is not continuous from theequator to the poles. In each hemisphere the existenceof three distinct tropopauses is generally agreedupon—one in the subtropical latitudes, one in middlelatitudes, and one in subpolar latitudes. They overlapeach other where they meet.

The tropopause is characterized by little or nochange in temperature with increasing altitude. Thecomposition of gases is about the same as that for thetroposphere. However, water vapor is found only invery minute quantities at the tropopause and above it.

STRATOSPHERE.—The stratosphere directlyoverlies the tropopause and extends to about 30 miles(160,000 ft or 48 kilometers). Temperature varies littlewith height in the stratosphere through the first 30,000feet (9,000 meters); however, in the upper portion thetemperature increases approximately linearly to valuesnearly equal to surface temperatures. This increase intemperature through this zone is attributed to thepresence of ozone that absorbs incoming ultravioletradiation.

STRATOPAUSE.—The stratopause is the top ofthe stratosphere. It is the zone marking another reversalwith increasing altitude (temperature begins todecrease with height).

MESOSPHERE.—The mesosphere is a layerapproximately 20 miles (100,000 ft or 32 kilometers)thick directly overlaying the stratopause. Thetemperature decreases with height.

MESOPAUSE.—The mesopause is the thinboundary zone between the mesosphere and thethermosphere. It is marked by a reversal oftemperatures; i.e., temperature again increases withaltitude.

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THERMOSPHERE.—The thermosphere, asecond region in which the temperature increases withheight, extends from the mesopause to the exosphere.

EXOSPHERE.—The very outer limit of Earth’satmosphere is regarded as the exosphere. It is the zonein which gas atoms are so widely spaced they rarelycollide with one another and have individual orbitsaround Earth.

Electrical Classification

The primary concern with the electricalclassification is the effect on communications andradar. The electrical classification outlines threezones—the troposphere, the ozonosphere, and theionosphere.

TROPOSPHERE.—The troposphere is importantto electrical transmissions because of the immensechanges in the density of the atmosphere that occur inthis layer. These density changes, caused by differencesin heat and moisture, affect the electronic emissionsthat travel through or in the troposphere. Electricalwaves can be bent or refracted when they pass throughthese different layers and the range and area ofcommunications may be seriously affected.

OZONOSPHERE.—This layer is nearlycoincident with the stratosphere. As was discussedearlier in this section, the ozone is found in this zone.Ozone is responsible for the increase in temperaturewith height in the stratosphere.

IONOSPHERE.—The ionosphere extends fromabout 40 miles (200,000 ft or 64 kilometers) to anindefinite height. Ionization of air molecules in thiszone provides conditions that are favorable for radiopropagation. This is because radio waves are sentoutward to the ionosphere and the ionized particlesreflect the radio waves back to Earth.

HEAT TRANSFER

The atmosphere is constantly gaining and losingheat. Wind movements are constantly transporting heatfrom one part of the world to another. It is due to theinequalities in gain and loss of heat that the air is almostconstantly in motion. Wind and weather directlyexpress the motions and heat transformations.

Methods

In meteorology, one is concerned with fourmethods of heat transfer. These methods are

conduction, convection, advection, and radiation. Heatis transferred from Earth directly the atmosphere byradiation, conduction, and advection. Heat istransferred within the atmosphere by radiation,conduction, and convection. Advection, a form ofconvection, is used in a special manner in meteorology.It is discussed as a separate method of heat transfer. Asradiation was discussed earlier in the unit, this sectioncovers conduction, convection, and advection.

CONDUCTION.—Conduction is the transfer ofheat from warmer to colder matter by contact. Althoughof secondary importance in heating the atmosphere, it isa means by which air close to the surface of Earth heatsduring the day and cools during the night.

CONVECTION.—Convection is the method ofheat transfer in a fluid resulting in the transport andmixing of the properties of that fluid. Visualize a pot ofboiling water. The water at the bottom of the pot isheated by conduction. It becomes less dense and rises.Cooler and denser water from the sides and the top ofthe pot rushes in and replaces the rising water. In time,the water is thoroughly mixed. As long as heat isapplied to the pot, the water continues to transfer heatby convection. The transfer of heat by convection inthis case applies only to what is happening to the waterin the pot. In meteorology, the term convection isnormally applied to vertical transport.

Convection occurs regularly in the atmosphere andis responsible for the development of air turbulence.Cumuliform clouds showers and thunderstorms occurwhen sufficient moisture is present and strong verticalconvection occurs. Vertical transfer of heat in theatmosphere (convection) works in a similar manner.Warmer, less dense air rises and is replaced bydescending cooler, denser air, which acquires heat.

Specific Heat

The specific heat of a substance shows how manycalories of heat it takes to raise the temperature of 1gram of that substance 1°C. Since it takes 1 calorie toraise the temperature of 1 gram of water 1°C, thespecific heat of water is 1. The specific heat of asubstance plays a tremendous role in meteorologybecause it is tied directly to temperature changes. Forinstance, the specific heat of earth in general is 0.33.This means it takes only 0.33 calorie to raise thetemperature of 1 gram of earth 1°C. Stated another way,earth heats and cools three times as fast as water.Therefore, assuming the same amount of energy(calories) is available, water heats (and cools) at a

1-17

slower rate than land does. The slower rate of heatingand cooling of water is the reason temperature extremesoccur over land areas while temperatures over waterareas are more consistent.

The specific heat of various land surfaces is alsodifferent, though the difference between one landsurface and another is not as great as between land andwater. Dry sand or bare rock has the lowest specificheat. Forest areas have the highest specific heat. Thisdifference in specific heat is another cause fordifferences in temperature for areas with different typesof surfaces even when they are only a few miles apart;this difference is important in understanding thehorizontal transport of heat (advection) on a smallerscale.

Advection is a form of convection, but inmeteorology it means the transfer of heat or otherproperties HORIZONTALLY. Convection is the termreserved for the VERTICAL transport of heat. In thismanual the words convection and advection are used tomean the vertical and horizontal transfer ofatmospheric properties, respectively.

Horizontal transfer of heat is achieved by motion ofthe air from one latitude and/or longitude to another. Itis of major importance in the exchange of air betweenpolar and equatorial regions. Since large masses of airare constantly on the move somewhere on Earth’ssurface and aloft, advection is responsible fortransporting more heat from place to place than anyother physical motion. Transfer of heat by advection isachieved not only by the transport of warm air, but alsoby the transport of water vapor that releases heat whencondensation occurs.

REVIEW QUESTIONS

Q1-11. What is the definition of Temperature?

Q1-12. What are 20 C converted to Fahrenheit?

Q1-13. Name the zones of the earth's atmosphere inascending order.

Q1-14. What are the four methods of heat transfer?

Q1-15. What is the horizontal transport of heatcalled?

MOISTURE

LEARNING OBJECTIVE: Describe howmoisture affects the atmosphere.

ATMOSPHERIC MOISTURE

More than two-thirds of Earth’s surface is coveredwith water. Water from this extensive source iscontinually evaporating into the atmosphere, coolingby various processes, condensing, and then falling tothe ground again as various forms of precipitation. Theremainder of Earth’s surface is composed of solid landof various and vastly different terrain features.Knowledge of terrain differences is very important inanalyzing and forecasting weather. The world’s terrainvaries from large-scale mountain ranges and deserts tominor rolling hills and valleys. Each type of terrainsignificantly influences local wind flow, moistureavailability, and the resulting weather.

Moisture in the atmosphere is found in threestates—solid, liquid, and gaseous. As a solid, it takesthe form of snow, hail, and ice pellets, frost, ice-crystalclouds, and ice-crystal fog. As a liquid, it is found asrain, drizzle, dew, and as the minute water dropletscomposing clouds of the middle and low stages as wellas fog. In the gaseous state, water forms as invisiblevapor. Vapor is the most important single element in theproduction of clouds and other visible weatherphenomena. The availability of water vapor for theproduction of precipitation largely determines theability of a region to support life.

The oceans are the primary source of moisture forthe atmosphere, but lakes, rivers, swamps, moist soil,snow, ice fields, and vegetation also furnish it. Moistureis introduced into the atmosphere in its gaseous state,and may then be carried great distances by the windbefore it is discharged as liquid or solid precipitation.

WATER VAPOR CHARACTERISTICS

There is a limit to the amount of water vapor thatair, at a given temperature, can hold. When this limit isreached, the air is said to be saturated. The higher the airtemperature, the more water vapor the air can holdbefore saturation is reached and condensation occurs.(See fig. 1-10.) For approximately every 20°F (11°C)increase in temperature between 0°F and 100°F (-18°Cand 38°C), the capacity of a volume of air to hold watervapor is about doubled. Unsaturated air, containing agiven amount of water vapor, becomes saturated if itstemperature decreases sufficiently; further coolingforces some of the water vapor to condense as fog,clouds, or precipitation.

1-18

The quantity of water vapor needed to producesaturation does not depend on the pressure of otheratmospheric gases. At a given temperature, the sameamount of water vapor saturates a given volume of air.This is true whether it be on the ground at a pressure of1000 mb or at an altitude of 17,000 ft (5,100 meters)with only 500 mb pressure, if the temperature is thesame. Since density decreases with altitude, a givenvolume of air contains less mass (grams) at 5,100meters than at the surface. In a saturated volume, therewould be more water vapor per gram of air at thisaltitude than at the surface.

Temperature

Although the quantity of water vapor in a saturatedvolume of atmosphere is independent of the airpressure, it does depend on the temperature. The higherthe temperature, the greater the tendency for liquidwater to turn into vapor. At a higher temperature,therefore, more vapor must be injected into a givenvolume before the saturated state is reached and dew orfog forms. On the other hand, cooling a saturated

volume of air forces some of the vapor to condense andthe quantity of vapor in the volume to diminish.

Condensation

Condensation occurs if moisture is added to the airafter it is saturated, or if cooling of the air reduces thetemperature below the saturation point. As shown infigure 1-11, the most frequent cause of condensation iscooling of the air from the following results: (a) airmoves over a colder surface, (b) air is lifted (cooled byexpansion), or (c) air near the ground is cooled at nightas a result of radiation cooling.

Pressure (Dalton’s Law)

The English physicist, John Dalton, formulated thelaws relative to the pressure of a mixture of gases. Oneof the laws states that the partial pressures of two ormore mixed gases (or vapors) are the same as if eachfilled the space alone. The other law states that the totalpressure is the sum of all the partial pressures of gasesand vapors present in an enclosure.

1-19

SATURATED CONDENSING

HEATCOOL

60 Fo

80 Fo

60 Fo

AGf0110

Figure 1-10.—Saturation of air depends on its temperature.

WARM COLDER COLDER

AIR MOVES IN OVERCOLDER SURFACE.

COOLED BYEXPANSION.

RADIATION COOLING

LIFTING

A B C

AGf0111

Figure 1-11.—Causes of condensation.

For instance, water vapor in the atmosphere isindependent of the presence of other gases. The vaporpressure is independent of the pressure of the dry gasesin the atmosphere and vice versa. However, the totalatmospheric pressure is found by adding all thepressures—those of the dry air and the water vapor.

TERMS

The actual amount of water vapor contained in theair is usually less than the saturation amount. Theamount of water vapor in the air is expressed in severaldifferent methods. Some of these principal methods aredescribed in the following portion of this section.

Relative Humidity

Although the major portion of the atmosphere isnot saturated, for weather analysis it is desirable to beable to say how near it is to being saturated. Thisrelationship is expressed as relative humidity. Therelative humidity of a volume of air is the ratio (inpercent) between the water vapor actually present andthe water vapor necessary for saturation at a giventemperature. When the air contains all of the watervapor possible for it to hold at its temperature, therelative humidity is 100 percent (See fig. 1-12). Arelative humidity of 50 percent indicates that the aircontains half of the water vapor that it is capable ofholding at its temperature.

Relative humidity is also defined as the ratio(expressed in percent) of the observed vapor pressure tothat required for saturation at the same temperature andpressure.

Relative humidity shows the degree of saturation,but it gives no clue to the actual amount of water vapor

in the air. Thus, other expressions of humidity areuseful.

Absolute Humidity

The mass of water vapor present per unit volume ofspace, usually expressed in grams per cubic meter, isknown as absolute humidity. It may be thought of as thedensity of the water vapor.

Specific Humidity

Humidity may be expressed as the mass of watervapor contained in a unit mass of air (dry air plus thewater vapor). It can also be expressed as the ratio of thedensity of the water vapor to the density of the air(mixture of dry air and water vapor). This is called thespecific humidity and is expressed in grams per gram orin grams per kilogram. This value depends upon themeasurement of mass, and mass does not change withtemperature and pressure. The specific humidity of aparcel of air remains constant unless water vapor isadded to or taken from the parcel. For this reason, airthat is unsaturated may move from place to place orfrom level to level, and its specific humidity remainsthe same as long as no water vapor is added or removed.However, if the air is saturated and cooled, some of thewater vapor must condense; consequently, the specifichumidity (which reflects only the water vapor)decreases. If saturated air is heated; its specifichumidity remains unchanged unless water vapor isadded to it. In this case the specific humidity increases.The maximum specific humidity that a parcel can haveoccurs at saturation and depends upon both thetemperature and the pressure. Since warm air can holdmore water vapor than cold air at constant pressure, thesaturation specific humidity at high temperatures is

1-20

THE DIFFERENCE BETWEENACTUAL TEMP. AND DEW POINTTEMP. IS AN INDICATION OFHOW CLOSE THE AIR IS TOSATURATION.

IF COOLED TO DEW POINTTEMP. OR ADDITIONALWATER VAPOR IS ADDEDTO SATURATED AIR,CONDENSATION OCCURS.

DRY AIR

WATER VAPOR

AIR TEMP

DEW POINT

RELATIVE HUMIDITY

40 60 80 80

80

80

8040

100 100100 100

60 60

50

OO O O O O

OO O O O O

% %% %%

F

F

AGf0112

Figure 1-12.—Relative humidity and dew point.

greater than at low temperatures. Also, since moist airis less dense than dry air at constant temperature, aparcel of air has a greater specific humidity atsaturation if the pressure is low than when the pressureis high.

Mixing Ratio

The mixing ratio is defined as the ratio of the massof water vapor to the mass of dry air and is expressed ingrams per gram or in grams per kilogram. It differsfrom specific humidity only in that it is related to themass of dry air instead of to the total dry air plus watervapor. It is very nearly equal numerically to specifichumidity, but it is always slightly greater. The mixingratio has the same characteristic properties as thespecific humidity. It is conservative (values do notchange) for atmospheric processes involving a changein temperature. It is non conservative for changesinvolving a gain or loss of water vapor.

Previously it was learned that air at any giventemperature can hold only a certain amount of watervapor before it is saturated. The total amount of vaporthat air can hold at any given temperature, by weightrelationship, is referred to as the saturation mixingratio. It is useful to note that the following relationshipexists between mixing ratio and relative humidity.Relative humidity is equal to the mixing ratio dividedby the saturation mixing ratio, multiplied by 100. If anytwo of the three components in this relationship areknown, the third may be determined by simplemathematics.

Dew Point

The dew point is the temperature that air must becooled, at constant pressure and constant water vaporcontent, in order for saturation to occur. The dew pointis a conservative and very useful element. Whenatmospheric pressure stays constant, the dew pointreflects increases and decreases in moisture in the air. Italso shows at a glance, under the same conditions, howmuch cooling of the air is required to condensemoisture from the air.

REVIEW QUESTIONS

Q1-16. Name the three states in which moisture in theatmosphere may be found.

Q1-17. What is the primary source of atmosphericmoisture?

Q1-18. What is the difference between relativehumidity and absolute humidity?

Q1-19. What is the definition of mixing ratio?

Q1-20. What information does the dew pointtemperature provide to meteorologists?

SUMMARY

In this chapter, we introduced the basicfundamentals of meteorology. It is important to have abasic knowledge of systems of measurement, how theearth and sun relate to each other, and how pressure,temperature and moisture are measured and calculated.An understanding of the basic fundamentals isnecessary before proceeding on to the next chapter.

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