Chapter 10Chapter 10

Weather Systems of Middle Weather Systems of Middle LatitudesLatitudes

Weather StudiesWeather Studies Introduction to Atmospheric ScienceIntroduction to Atmospheric Science

American Meteorological SocietyAmerican Meteorological Society

Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

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Case-In-PointCase-In-Point Extra-tropical cyclones are major weather Extra-tropical cyclones are major weather

makers in middle and high latitudesmakers in middle and high latitudes In 1703, Daniel Defoe was the first to In 1703, Daniel Defoe was the first to

propose that storms generally track from propose that storms generally track from west to east in middle latitudeswest to east in middle latitudes

In 1743, Benjamin Franklin was the first In 1743, Benjamin Franklin was the first American to discover that storms usually American to discover that storms usually move in an easterly or northeasterly move in an easterly or northeasterly directiondirection– Based on observations, he concluded that wind Based on observations, he concluded that wind

direction in a storm was not an indication of the direction in a storm was not an indication of the storm’s direction of movementstorm’s direction of movement

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Driving QuestionDriving Question

What systems shape the weather of the What systems shape the weather of the middle latitudes?middle latitudes?– Middle latitudes extend from Tropics of Cancer Middle latitudes extend from Tropics of Cancer

and Capricorn, poleward to the Arctic/Antarctic and Capricorn, poleward to the Arctic/Antarctic CirclesCircles

– Weather is particularly dynamic because of the Weather is particularly dynamic because of the migration of cyclones and anticyclones migration of cyclones and anticyclones embedded in the prevailing westerliesembedded in the prevailing westerlies

– This chapter examines:This chapter examines: Air masses, fronts, cyclones, and anticyclonesAir masses, fronts, cyclones, and anticyclones Local and regional circulation systemsLocal and regional circulation systems

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Air MassesAir Masses An air mass is a huge expanse of air covering An air mass is a huge expanse of air covering

thousands of square kilometers, and is relatively thousands of square kilometers, and is relatively uniform horizontally in temperature and water uniform horizontally in temperature and water vapor concentration (humidity)vapor concentration (humidity)

Abbreviations for air mass types:Abbreviations for air mass types:– Cold (polar or Cold (polar or PP) or warm (tropical or ) or warm (tropical or TT))– Dry (continental or Dry (continental or cc) or humid (maritime or ) or humid (maritime or mm))– Arctic (Arctic (AA) air) air

Air mass source regions have nearly Air mass source regions have nearly homogeneous surface characteristics over a broad homogeneous surface characteristics over a broad area with little topographic reliefarea with little topographic relief– The air mass stays over the source region for an The air mass stays over the source region for an

extended period, and takes on the characteristics of the extended period, and takes on the characteristics of the source regionsource region

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North American Types North American Types and Source Regionsand Source Regions

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North American Air MassesNorth American Air Masses

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Air MassesAir Masses Modification of Air MassesModification of Air Masses

– Air masses eventually move out of their source Air masses eventually move out of their source regionregion

– As they move, their properties are modified by As they move, their properties are modified by the surface they pass overthe surface they pass over

– Air mass modification occurs from:Air mass modification occurs from: Exchange of heat or moisture, or both, with the Exchange of heat or moisture, or both, with the

surface over which the air mass travelssurface over which the air mass travels Radiational heating or coolingRadiational heating or cooling Adiabatic heating or cooling associated with large-Adiabatic heating or cooling associated with large-

scale vertical motionscale vertical motion

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Air MassesAir Masses Modification of Air Masses, continuedModification of Air Masses, continued

– In winter, as a In winter, as a cPcP air mass travels southeastward from air mass travels southeastward from Canada into the lower 48-states, its temperature usually Canada into the lower 48-states, its temperature usually modifies rapidlymodifies rapidly By the time it arrives in the southern states, temperatures will not By the time it arrives in the southern states, temperatures will not

usually drop much below freezingusually drop much below freezing The sun warms snow-free ground, and the warmer ground heats The sun warms snow-free ground, and the warmer ground heats

the bottom of the air mass. Heat is then distributed vertically.the bottom of the air mass. Heat is then distributed vertically.

– A similar process of heating and destabilization occurs A similar process of heating and destabilization occurs when a when a cPcP air mass crosses the East Coast and moves air mass crosses the East Coast and moves over the western Atlantic. Evaporation from the sea over the western Atlantic. Evaporation from the sea surface leads to extensive areas of low clouds and fogsurface leads to extensive areas of low clouds and fog

– cPcP traveling over snow-covered ground experiences less traveling over snow-covered ground experiences less modificationmodification Much of the incoming solar radiation is reflected rather than being Much of the incoming solar radiation is reflected rather than being

absorbedabsorbed

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Air MassesAir Masses Modification of Air Masses, continuedModification of Air Masses, continued

– Tropical air masses modify less than polar massesTropical air masses modify less than polar masses They are often warmer than the ground they travel overThey are often warmer than the ground they travel over The bottom of the air mass cools, and stabilizesThe bottom of the air mass cools, and stabilizes Convective currents are suppressedConvective currents are suppressed If a tropical air mass moves over a warmer surface, the air mass If a tropical air mass moves over a warmer surface, the air mass

can become even warmercan become even warmer

– Air masses undergo significant modification through Air masses undergo significant modification through orographic uplifting (e.g., orographic uplifting (e.g., mPmP air mass sweeping inland air mass sweeping inland from the Pacific Ocean)from the Pacific Ocean) Rising air cools adiabatically, condensation/deposition occur, Rising air cools adiabatically, condensation/deposition occur,

and precipitation is triggered on the windward slopesand precipitation is triggered on the windward slopes Descent on the leeward side leads to adiabatic warming and Descent on the leeward side leads to adiabatic warming and

cloud dissipationcloud dissipation Air mass emerges considerably milder and drier (e.g., modified Air mass emerges considerably milder and drier (e.g., modified

Pacific air)Pacific air)

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Frontal WeatherFrontal Weather A front is a narrow zone of transition between air masses that A front is a narrow zone of transition between air masses that

differ in densitydiffer in density– Density differences are usually due to temperature contrasts, hence the Density differences are usually due to temperature contrasts, hence the

names cold fronts and warm frontsnames cold fronts and warm fronts– Density differences may also be caused by humidity contrastsDensity differences may also be caused by humidity contrasts– A front’s transition zone may be 100+ km and a line representing a front A front’s transition zone may be 100+ km and a line representing a front

on a weather map is drawn along the warm edge of the zoneon a weather map is drawn along the warm edge of the zone A front is also associated with a trough in the sea-level A front is also associated with a trough in the sea-level

pressure pattern, a corresponding wind shift, and converging pressure pattern, a corresponding wind shift, and converging windswinds

When air masses meet at fronts, the colder, denser air forces When air masses meet at fronts, the colder, denser air forces the warmer, less dense air to risethe warmer, less dense air to rise– This induces adiabatic cooling and often cloud/precipitation developmentThis induces adiabatic cooling and often cloud/precipitation development

The slope of the front influences the types of clouds that formThe slope of the front influences the types of clouds that form– Cold and warm fronts have different slopes associated with themCold and warm fronts have different slopes associated with them

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Frontal WeatherFrontal Weather Stationary FrontStationary Front

– A front that exhibits essentially no lateral motionA front that exhibits essentially no lateral motion– This often happens along the Front Range of the Rocky This often happens along the Front Range of the Rocky

Mountains when a shallow pool of polar air surges south Mountains when a shallow pool of polar air surges south over the plains and the leading edge is too shallow to over the plains and the leading edge is too shallow to cross the mountains. Milder air remains in the Great cross the mountains. Milder air remains in the Great Basin to the west of the Rockies.Basin to the west of the Rockies.

– May also develop when a preexisting front becomes May also develop when a preexisting front becomes parallel to the upper-level flow pattern or along a parallel to the upper-level flow pattern or along a boundary in the surface temperature patternboundary in the surface temperature pattern

– Typical frontTypical front Slopes from Earth’s surface towards denser airSlopes from Earth’s surface towards denser air Lies in a trough in the pressure patternLies in a trough in the pressure pattern Wind changes direction rather abruptly across the frontWind changes direction rather abruptly across the front May have broad region of clouds and precipitation (e.g., May have broad region of clouds and precipitation (e.g.,

overrunning)overrunning)

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Stationary FrontStationary Front

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Frontal WeatherFrontal Weather Warm FrontWarm Front

– Warm air advances while cold air retreatsWarm air advances while cold air retreats– Overall characteristics very similar to a stationary frontOverall characteristics very similar to a stationary front– As a warm front approaches:As a warm front approaches:

Clouds thicken and become lower in altitudeClouds thicken and become lower in altitude Sequence is cirrus, cirrostratus, altostratus, nimbostratus, Sequence is cirrus, cirrostratus, altostratus, nimbostratus,

and stratus when the advancing warm air is relatively and stratus when the advancing warm air is relatively stablestable

Initial cirrus appearance may be more than 1000 km (620 Initial cirrus appearance may be more than 1000 km (620 mi) ahead of the frontmi) ahead of the front

Just ahead of the front, steady precipitation usually gives Just ahead of the front, steady precipitation usually gives way to drizzle and sometimes frontal fogway to drizzle and sometimes frontal fog

If advancing warm air is unstable, more vigorous uplift If advancing warm air is unstable, more vigorous uplift can occur with thunderstorms embedded in the can occur with thunderstorms embedded in the overrunning zoneoverrunning zone

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Warm FrontWarm Front

Cirrus clouds

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Frontal WeatherFrontal Weather Cold FrontCold Front

– Colder air displaces warmer airColder air displaces warmer air– In North America in winter, the temperature contrast In North America in winter, the temperature contrast

along a cold front is usually greater than across a warm along a cold front is usually greater than across a warm or stationary frontor stationary front

– In summer, temperatures on either side of the front may In summer, temperatures on either side of the front may be essentially the samebe essentially the same Density contrasts arise because of humidity differencesDensity contrasts arise because of humidity differences

– The slope on a cold front is much steeper than the The slope on a cold front is much steeper than the slope on a warm frontslope on a warm front Uplift is confined to a narrow area at or near the cold Uplift is confined to a narrow area at or near the cold

front’s leading edgefront’s leading edge If the warm air is unstable, thunderstorms may form and If the warm air is unstable, thunderstorms may form and

a squall line can developa squall line can develop If the warm air is relatively stable, nimbostratus and If the warm air is relatively stable, nimbostratus and

altostratus may formaltostratus may form

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Cold FrontCold Front

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Advancing Back-Door Cold FrontAdvancing Back-Door Cold Front

A cold front generally trails south or southwestward from the center of A cold front generally trails south or southwestward from the center of an extra-tropical cyclone. Back-door cold fronts move south along the an extra-tropical cyclone. Back-door cold fronts move south along the eastern side of the Appalachian Mountains.eastern side of the Appalachian Mountains.

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Frontal WeatherFrontal Weather Occluded FrontsOccluded Fronts

– Typically form late in a cyclone’s life cycle as it moves into Typically form late in a cyclone’s life cycle as it moves into colder aircolder air

– Faster moving cold front catches up with the warm frontFaster moving cold front catches up with the warm front– There are 3 types of occlusions, distinguished by the There are 3 types of occlusions, distinguished by the

temperature contrast between the air behind the cold front temperature contrast between the air behind the cold front and ahead of the warm frontand ahead of the warm front Cold occlusionCold occlusion

– Air behind cold front colder than cool air ahead of warm frontAir behind cold front colder than cool air ahead of warm front– Like a cold front at the surface but, with less air mass contrastLike a cold front at the surface but, with less air mass contrast

Warm occlusionWarm occlusion– Air behind cold front is not as cold as the air ahead of the warm frontAir behind cold front is not as cold as the air ahead of the warm front– Like a warm front at the surfaceLike a warm front at the surface

Neutral occlusionNeutral occlusion– Little difference between air massesLittle difference between air masses– Marked by a trough, wind shift line, band of cloudiness & precipitationMarked by a trough, wind shift line, band of cloudiness & precipitation

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Cold-Type OcclusionCold-Type Occlusion

Air behind advancing Air behind advancing cold front colder than cold front colder than cool air ahead of warm cool air ahead of warm frontfront

More common in More common in eastern North America, eastern North America, where the colder air where the colder air follows behind the front follows behind the front on northwest windson northwest winds

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Warm-Type OcclusionWarm-Type Occlusion

Air behind the advancing Air behind the advancing cold front is not as cold as cold front is not as cold as the air ahead of the warm the air ahead of the warm frontfront

Occurs in northerly Occurs in northerly portions of western coasts, portions of western coasts, such as in Europe or the such as in Europe or the Pacific Northwest, where Pacific Northwest, where mPmP air is behind the cold air is behind the cold frontfront

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Air MassesAir Masses

SummarySummary– Fronts are characterized based on the movement of the Fronts are characterized based on the movement of the

cold air masscold air mass– Clouds and precipitation may develop along fronts when Clouds and precipitation may develop along fronts when

there is a significant density contrast between air there is a significant density contrast between air masses and there is an adequate supply of water vapormasses and there is an adequate supply of water vapor

– Properties that define a front are differences in Properties that define a front are differences in temperature and humidity, wind shift, convergence, and temperature and humidity, wind shift, convergence, and a pressure trougha pressure trough

– Frontogensis: front forms or grows strongerFrontogensis: front forms or grows stronger– Frontolysis: front weakensFrontolysis: front weakens

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Extra-tropical CyclonesExtra-tropical Cyclones

The extra-tropical cyclone (also called a low-pressure system or low), is a The extra-tropical cyclone (also called a low-pressure system or low), is a major weather maker of middle and high latitudes. Surface winds blow major weather maker of middle and high latitudes. Surface winds blow counterclockwise and inward. Surface winds converge, air rises, counterclockwise and inward. Surface winds converge, air rises, expands, and cools, resulting in clouds and precipitation.expands, and cools, resulting in clouds and precipitation.

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Extra-tropical CyclonesExtra-tropical Cyclones

The comma-shaped cloud pattern is The comma-shaped cloud pattern is characteristic of a well-developed extra-characteristic of a well-developed extra-tropical cyclone.tropical cyclone.

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Extra-tropicalExtra-tropicalCyclonesCyclones

Life CycleLife Cycle– Norwegian cyclone model: conceptual model originally Norwegian cyclone model: conceptual model originally

developed around WWI still closely approximates our developed around WWI still closely approximates our current understandingcurrent understanding

– (A) Incipient cyclone: Cyclogenesis (birth of a cyclone) (A) Incipient cyclone: Cyclogenesis (birth of a cyclone) usually takes place along the polar front directly under usually takes place along the polar front directly under an area of strong horizontal divergence in the upper an area of strong horizontal divergence in the upper tropospheretroposphere Air pressure at the bottom of the air column falls, a horizontal air Air pressure at the bottom of the air column falls, a horizontal air

pressure gradient develops, and cyclonic circulation beginspressure gradient develops, and cyclonic circulation begins Westerlies aloft steer and support the cyclone as it progresses Westerlies aloft steer and support the cyclone as it progresses

through its life cyclethrough its life cycle West of the low center, the polar front pushes southeast as a West of the low center, the polar front pushes southeast as a

cold front. East of the low, the polar front advances north as a cold front. East of the low, the polar front advances north as a warm front.warm front.

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Extra-tropicalExtra-tropicalCyclonesCyclones

Life CycleLife Cycle– (B) Wave cyclone: In this stage, the central pressure (B) Wave cyclone: In this stage, the central pressure

continues to drop and winds strengthen due to an continues to drop and winds strengthen due to an increased pressure gradient. The upper-level trough increased pressure gradient. The upper-level trough deepens while remaining west of the surface low center.deepens while remaining west of the surface low center. Warm sector becomes better definedWarm sector becomes better defined Fronts form a pronounced wave pattern and comma cloud is Fronts form a pronounced wave pattern and comma cloud is

seen in satellite imagesseen in satellite images Extensive stratiform cloudiness appears north of the warm frontExtensive stratiform cloudiness appears north of the warm front Cyclone moves eastward or northeastward at 40-55 km per hr Cyclone moves eastward or northeastward at 40-55 km per hr

(25-35 mph)(25-35 mph)

warm sector

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Extra-tropicalExtra-tropicalCyclonesCyclones

Life CycleLife Cycle– (C) Beginning of occlusion(C) Beginning of occlusion

Faster moving cold front advances on the warm frontFaster moving cold front advances on the warm front Warm sector area diminishes and occluded front begins to formWarm sector area diminishes and occluded front begins to form Upper level pattern shows closed circulation and is directly over Upper level pattern shows closed circulation and is directly over

the surface low (vertically stacked)the surface low (vertically stacked) Dry slot separates the cold front cloud band from the comma Dry slot separates the cold front cloud band from the comma

cloudcloud Cyclone moves slower at approximately 30 km per hr (20 mph)Cyclone moves slower at approximately 30 km per hr (20 mph)

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Extra-tropicalExtra-tropicalCyclonesCyclones

Life CycleLife Cycle– (D) Bent-back occlusion(D) Bent-back occlusion

Surface low may become detached from the westerly Surface low may become detached from the westerly steering flow and the occluded front is drawn around steering flow and the occluded front is drawn around the low centerthe low center

Warm sector is detached from cyclone centerWarm sector is detached from cyclone center Triple point favors development of a secondary Triple point favors development of a secondary

cyclonecyclone Eventually the cyclone weakens (cyclolysis)Eventually the cyclone weakens (cyclolysis)

triple point

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Extra-tropical CyclonesExtra-tropical Cyclones

Entire cycle can occur over several days, or a Entire cycle can occur over several days, or a much shorter periodmuch shorter period– Speed of development depends on upper air supportSpeed of development depends on upper air support

Weak divergence aloft will cause poorly defined systemsWeak divergence aloft will cause poorly defined systems

– Sometimes cloudiness and precipitation occur with an Sometimes cloudiness and precipitation occur with an upper-level or surface trough, which is not associated upper-level or surface trough, which is not associated with a closed surface cyclonic circulationwith a closed surface cyclonic circulation

When upper-level conditions are ideal, the entire When upper-level conditions are ideal, the entire life cycle can occur in less than 36 hourslife cycle can occur in less than 36 hours

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Extra-tropical CyclonesExtra-tropical Cyclones Cyclone BombCyclone Bomb

– This is the term applied to a rapidly intensifying cyclone, This is the term applied to a rapidly intensifying cyclone, and is defined as a central pressure drop of at least 24 and is defined as a central pressure drop of at least 24 mb in 24 hoursmb in 24 hours

– Few cyclones meet this criteria, and most that do occur Few cyclones meet this criteria, and most that do occur in winter over a warm ocean surface current (e.g., Gulf in winter over a warm ocean surface current (e.g., Gulf Stream)Stream)

Conveyor Belt ModelConveyor Belt Model– This is an alternate, 3-D model to the steps discussed This is an alternate, 3-D model to the steps discussed

previously (Norwegian cyclone model)previously (Norwegian cyclone model)– Combines horizontal and vertical air motionsCombines horizontal and vertical air motions– Depicts the circulation in a mature cyclone in terms of Depicts the circulation in a mature cyclone in terms of

three broad interacting systems called conveyor belts, three broad interacting systems called conveyor belts, which transport air with certain properties from one which transport air with certain properties from one location to anotherlocation to another Belts are (1) warm and humid, (2) cold, and (3) dryBelts are (1) warm and humid, (2) cold, and (3) dry

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Extra-tropical CyclonesExtra-tropical Cyclones Conveyor Belt Model, continuedConveyor Belt Model, continued

– (1) Warm and humid conveyor belt originates in the (1) Warm and humid conveyor belt originates in the cyclone’s warm sectorcyclone’s warm sector Ascends slightly as it flows northward in the warm sector at low Ascends slightly as it flows northward in the warm sector at low

levels and then ascends more rapidly over the warm frontlevels and then ascends more rapidly over the warm front Helps explain the broad region of clouds/precipitation north of the Helps explain the broad region of clouds/precipitation north of the

warm frontwarm front

– (2) Cold conveyor belt originates north of the warm front (2) Cold conveyor belt originates north of the warm front Ascends as it progresses toward the westAscends as it progresses toward the west Forms the comma cloud and produces precipitationForms the comma cloud and produces precipitation Turns clockwise at upper levels and follows westerly flow aloftTurns clockwise at upper levels and follows westerly flow aloft

– (3) Dry conveyor belt(3) Dry conveyor belt This air originates high in the troposphere and low stratosphere This air originates high in the troposphere and low stratosphere

upstream of the upper-level troughupstream of the upper-level trough One branch descends southward behind the cold front; the other One branch descends southward behind the cold front; the other

forms the dry slot that separates the head & tail of the comma cloudforms the dry slot that separates the head & tail of the comma cloud

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Conveyor Belt ModelConveyor Belt Model

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Extra-tropical CyclonesExtra-tropical CyclonesCyclone WeatherCyclone Weather

Figure below represents an intensifying cyclone in the Upper MidwestFigure below represents an intensifying cyclone in the Upper Midwest Four sectors about the low center:Four sectors about the low center:

– Strong cold air advection, stratiform clouds, and non-convective Strong cold air advection, stratiform clouds, and non-convective precipitation precipitation northwestnorthwest of the low of the low

– Cold front south of low is accompanied by convective precipitation. Sinking Cold front south of low is accompanied by convective precipitation. Sinking air and mostly clear skies characterize the air and mostly clear skies characterize the southwestsouthwest sector behind the sector behind the cold front.cold front.

– The mildest air is in the The mildest air is in the southeastsoutheast (warm) sector of the cyclone (warm) sector of the cyclone– An extensive overrunning zone is found to the An extensive overrunning zone is found to the northeastnortheast of the low center of the low center

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Principal Cyclone TracksPrincipal Cyclone Tracks As a general rule, the cyclone center moves forward in the same direction As a general rule, the cyclone center moves forward in the same direction

and at about one-half the speed of the 500-mb winds and at about one-half the speed of the 500-mb winds Principal storm tracks tend to converge toward the northeastPrincipal storm tracks tend to converge toward the northeast Storm tracks appear to originate just east of the Rocky Mountains, but Storm tracks appear to originate just east of the Rocky Mountains, but

actually form over the Pacific Ocean near Alaskaactually form over the Pacific Ocean near Alaska– As a cyclone travels over the mountains, it often loses its identity, but reforms As a cyclone travels over the mountains, it often loses its identity, but reforms

over the Great Plainsover the Great Plains Nor’easters often intensify off the North Carolina coast and track toward Nor’easters often intensify off the North Carolina coast and track toward

the northeast along the East Coastthe northeast along the East Coast– 2 motions exist2 motions exist

Movement of the cyclone along the coastMovement of the cyclone along the coast Counterclockwise flow of winds around the storm center; winds in northeast sector Counterclockwise flow of winds around the storm center; winds in northeast sector

of the cyclone blow from the northeast (gives the name “nor-easter”)of the cyclone blow from the northeast (gives the name “nor-easter”)– Some may become powerful systems; drawing copious amounts of water Some may become powerful systems; drawing copious amounts of water

vapor from the ocean and producing large amounts of precipitation over a vapor from the ocean and producing large amounts of precipitation over a broad areabroad area

Generally, cyclones that form in the south yield more precipitation Generally, cyclones that form in the south yield more precipitation because they have access to greater amounts of because they have access to greater amounts of mTmT air air

Cyclogenesis is more frequent in the winter when the mean position of Cyclogenesis is more frequent in the winter when the mean position of the polar front and jet stream shift southwardthe polar front and jet stream shift southward

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Principal Cyclone TracksPrincipal Cyclone Tracks

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Cold Side/Warm SideCold Side/Warm Side– Storm track determines Storm track determines

weather at points on the weather at points on the groundground

– Track A puts Chicago on the Track A puts Chicago on the warm side with passage of warm side with passage of the warm and cold frontsthe warm and cold fronts

– Track B puts Chicago on the Track B puts Chicago on the cold side with no frontal cold side with no frontal passagepassage

– Table summarizes the Table summarizes the general sequence of weather general sequence of weather conditions at Chicagoconditions at Chicago

Extra-tropical CyclonesExtra-tropical Cyclones

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Extra-tropical CyclonesExtra-tropical Cyclones

Winter StormsWinter Storms– An extra-tropical cyclone that produces any combination An extra-tropical cyclone that produces any combination

of frozen or freezing precipitationof frozen or freezing precipitation– An associated hazard is a cold wave, which often An associated hazard is a cold wave, which often

follows a winter stormfollows a winter storm– Necessary ingredients include cold air (typically brought Necessary ingredients include cold air (typically brought

in by a sprawling cold high to the north), a moisture in by a sprawling cold high to the north), a moisture supply, and uplift mechanismssupply, and uplift mechanisms

– A major storm requires warm and humid air brought A major storm requires warm and humid air brought northwardnorthward

– A storm moving to the northeast produces heaviest A storm moving to the northeast produces heaviest snow to the north and west of the low centersnow to the north and west of the low center

– Blizzard: a severe storm characterized by high winds Blizzard: a severe storm characterized by high winds and reduced visibility due to falling or blowing snowand reduced visibility due to falling or blowing snow

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Colorado-track Winter Storm SystemColorado-track Winter Storm System

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Extra-tropical CyclonesExtra-tropical CyclonesCold and Warm Core SystemsCold and Warm Core Systems

An occluded cyclone is a An occluded cyclone is a cold-core systemcold-core system– Lowest temperatures occur in a Lowest temperatures occur in a

column just above the surface column just above the surface lowlow

– Depth of low increases with Depth of low increases with altitudealtitude

– Cyclonic circulation prevails Cyclonic circulation prevails throughout the troposphere and throughout the troposphere and is most intense at high altitudesis most intense at high altitudes

– The requirement that thickness The requirement that thickness (mean temperature) be lowest (mean temperature) be lowest at the low center produces the at the low center produces the classic isobar patternclassic isobar pattern

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Extra-tropical CyclonesExtra-tropical CyclonesCold- and Warm-Core SystemsCold- and Warm-Core Systems

A non-occluded cyclone A non-occluded cyclone is a warm-core systemis a warm-core system– Lowest temperatures are Lowest temperatures are

northwest of the cyclone’s northwest of the cyclone’s center, and highest center, and highest temperatures are to the temperatures are to the southeastsoutheast

– Low aloft is displaced to Low aloft is displaced to cold side of the stormcold side of the storm

– The system tilts with The system tilts with altitudealtitude

– Upper-level low lags Upper-level low lags behind surface lowbehind surface low

Vertical cross-section of a low from Vertical cross-section of a low from northwest (cold) to southeast (warm)northwest (cold) to southeast (warm)

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Extra-tropical CyclonesExtra-tropical CyclonesCold and Warm Core SystemsCold and Warm Core Systems

Warm-core cyclone (thermal low)Warm-core cyclone (thermal low)– Stationary, have no fronts, and are generally Stationary, have no fronts, and are generally

associated with fair weatherassociated with fair weather– From over a broad expanse of arid/semiarid From over a broad expanse of arid/semiarid

land in response to intense solar heating of the land in response to intense solar heating of the groundground Hot surface heats the overlying air and lowers the Hot surface heats the overlying air and lowers the

density of the air column enough for a low to formdensity of the air column enough for a low to form

– Usually very shallowUsually very shallow– Anticyclone aloft overlies lowAnticyclone aloft overlies low

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AnticyclonesAnticyclones

In anticyclones, subsiding air and diverging surface winds In anticyclones, subsiding air and diverging surface winds favor formation of a uniform air mass, no fronts, and favor formation of a uniform air mass, no fronts, and generally fair skiesgenerally fair skies

Arctic and Polar Highs (cold-core anticylone)Arctic and Polar Highs (cold-core anticylone)– Labeled either a polar high (Labeled either a polar high (cPcP air) or arctic high ( air) or arctic high (AA air) air)

and are products of extreme radiational cooling, often and are products of extreme radiational cooling, often over snow-covered landover snow-covered land

– Clockwise circulation weakens with altitude, and may Clockwise circulation weakens with altitude, and may reversereverse

– Usually has a cold trough overlying itUsually has a cold trough overlying it– These exert the highest pressure in winterThese exert the highest pressure in winter– They are extremely stable, with an inversion in the lower They are extremely stable, with an inversion in the lower

km or sokm or so– Interact with the circulation of an extra-tropical cyclone Interact with the circulation of an extra-tropical cyclone

by helping to maintain and strengthen the temperature by helping to maintain and strengthen the temperature contrast along the cyclone’s cold frontcontrast along the cyclone’s cold front

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AnticyclonesAnticyclones

Warm High (warm-core anticyclone)Warm High (warm-core anticyclone)– Forms south of the polar front and consists of Forms south of the polar front and consists of

extensive areas of subsiding warm and dry airextensive areas of subsiding warm and dry air– These strengthen with altitudeThese strengthen with altitude– Examples are Bermuda-Azores high and Examples are Bermuda-Azores high and

systems that may develop over the interior of systems that may develop over the interior of North America, especially in summerNorth America, especially in summer

– The greater mass of air over the anticyclone The greater mass of air over the anticyclone center (related to a higher tropopause) is center (related to a higher tropopause) is responsible for the high surface pressureresponsible for the high surface pressure

– A cold-core anticyclone can become warm-core A cold-core anticyclone can become warm-core as it moves south and modifiesas it moves south and modifies

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AnticyclonesAnticyclones Anticyclone WeatherAnticyclone Weather

– Fair weather system because surface winds blowing in Fair weather system because surface winds blowing in a clockwise and outward direction (Northern a clockwise and outward direction (Northern Hemisphere) induce subsidence over a broad areaHemisphere) induce subsidence over a broad area

– Arctic highs produce the lowest temperatures of winterArctic highs produce the lowest temperatures of winter– A stalling warm anticyclone can lead to drought and A stalling warm anticyclone can lead to drought and

excessive summer heatexcessive summer heat– A weak horizontal air pressure gradient near the center A weak horizontal air pressure gradient near the center

leads to intense nighttime radiational coolingleads to intense nighttime radiational cooling– Ahead of an anticyclone, there may be strong northwest Ahead of an anticyclone, there may be strong northwest

winds ushering in polar or arctic airwinds ushering in polar or arctic air May bring heavy lake-effect snows to the lee side of the lakesMay bring heavy lake-effect snows to the lee side of the lakes In the summer, the most noticeable effect is not a lowering of In the summer, the most noticeable effect is not a lowering of

temperatures, but a lowering of humiditytemperatures, but a lowering of humidity– Highs may become entrenched east of the Rockies in Highs may become entrenched east of the Rockies in

summer, and form blocking highssummer, and form blocking highs High temperatures and eventually drought resultHigh temperatures and eventually drought result

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Local and Regional Circulation SystemsLocal and Regional Circulation SystemsLand and Sea (or Lake) BreezesLand and Sea (or Lake) Breezes

Sea BreezeSea Breeze– Under exposure to the same intensity Under exposure to the same intensity

of solar radiation, the land surface of solar radiation, the land surface warms more than the water surfacewarms more than the water surface

– Highest pressure over water, and Highest pressure over water, and cool breeze sweeps inlandcool breeze sweeps inland

– Shallow circulation has maximum Shallow circulation has maximum strength in mid-afternoonstrength in mid-afternoon

– Uplift may lead to thunderstormsUplift may lead to thunderstorms

Land BreezeLand Breeze– By late evening, winds blow offshore By late evening, winds blow offshore

due to a reversal in the heat due to a reversal in the heat differential between land and waterdifferential between land and water

– Obtains maximum strength around Obtains maximum strength around sunrise but is weaker than a sea sunrise but is weaker than a sea breezebreeze

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Local and Regional Circulation SystemsLocal and Regional Circulation SystemsChinook WindsChinook Winds

A relatively warm and dry wind A relatively warm and dry wind that develops when air that develops when air descending the leeward slopes descending the leeward slopes of a mountain range is of a mountain range is adiabatically compressedadiabatically compressed

Strong winds cause stable air in Strong winds cause stable air in the lower troposphere to ascend the lower troposphere to ascend on the windward sideon the windward side

On the leeward side, stable air On the leeward side, stable air descends to the original altitude, descends to the original altitude, and the larger scale of and the larger scale of circulation causes further circulation causes further descentdescent

Called Santa Ana winds in Called Santa Ana winds in southern Californiasouthern California– The figure is a schematic The figure is a schematic

representation of the surface representation of the surface weather pattern that favors weather pattern that favors developmentdevelopment

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Local and Regional Circulation SystemsLocal and Regional Circulation SystemsChinook WindsChinook Winds

Boulder, CO, situated in Boulder, CO, situated in the foothills of the Rocky the foothills of the Rocky Mountains, experiences Mountains, experiences particularly strong and particularly strong and destructive downslope destructive downslope winds, sometimes gusting winds, sometimes gusting to 160 km per hr (100 to 160 km per hr (100 mph)mph)

On average, the On average, the community sustains about community sustains about $1 million in property $1 million in property damage each year due to damage each year due to these windsthese winds

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Local and Regional Circulation SystemsLocal and Regional Circulation SystemsDesert WindsDesert Winds

Hot surfaces (i.e., deserts) may develop superadiabatic Hot surfaces (i.e., deserts) may develop superadiabatic lapse rates in the lowest levels of the atmospherelapse rates in the lowest levels of the atmosphere– These are highly unstable, and generate vigorous upwelling and These are highly unstable, and generate vigorous upwelling and

gusty surface winds, but very few clouds formgusty surface winds, but very few clouds form A dust devil is a whirling mass of dust-laden air formed by A dust devil is a whirling mass of dust-laden air formed by

localized hot spotlocalized hot spot– Air is heated, and rises rapidlyAir is heated, and rises rapidly– Cooler surface winds converge on the hot spotCooler surface winds converge on the hot spot– Horizontal wind shear causes the column of rising hot air to spin Horizontal wind shear causes the column of rising hot air to spin

about a nearly vertical axisabout a nearly vertical axis– Dust and debris are picked up, making these visible to altitudes Dust and debris are picked up, making these visible to altitudes

topping 900 m (3000 ft)topping 900 m (3000 ft)– May cause damage, as some have winds as higher than 75 km per May cause damage, as some have winds as higher than 75 km per

hr (45 mph)hr (45 mph) Strong thunderstorm downdrafts may generate dust storms Strong thunderstorm downdrafts may generate dust storms

known as a haboobknown as a haboob

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Local and Regional Circulation SystemsLocal and Regional Circulation SystemsMountain or Valley BreezesMountain or Valley Breezes

Valley BreezeValley Breeze– Bare valley walls absorb solar radiation

and heat the surrounding air– Cooler, denser air over the valley sinks

and air adjacent to the valley walls blows upslope

– Cumulus clouds may form near summit– Best developed between late morning

and sunset Mountain BreezeMountain Breeze

– Bare valley walls are chilled by radiational cooling and cool the surrounding air

– Colder, denser air near the valley walls sinks and gusty breeze blows downslope

– Fog or low stratus clouds may form in the valley