WX-201Chapter 12

Air Masses and Fronts

Air Masses & FrontsAir Masses & Fronts

This chapter discusses:This chapter discusses:

1.1. Classification of 4 North American air masses based on Classification of 4 North American air masses based on cold or warm land mass and ocean origincold or warm land mass and ocean origin

2.2. Air mass delineation and movement along stationary, Air mass delineation and movement along stationary, cold, warm, and occluded frontscold, warm, and occluded fronts

Regional Weather PatternsRegional Weather Patterns

Figure 12.1Figure 12.1

Surface maps of US Surface maps of US temperature, dew point, temperature, dew point, and pressure reflect and pressure reflect synoptic trends.synoptic trends.

In this image, nearly In this image, nearly every station around the every station around the high pressure high pressure anticyclone reports anticyclone reports cold, dry air, suggesting cold, dry air, suggesting the air mass formed in the air mass formed in a common region.a common region.

Source Regions for Air Masses

US Source Regions & US Source Regions & ClassificationClassification

Figure 12.2Figure 12.2

Air masses of similar temperature and humidity form Air masses of similar temperature and humidity form above flat, uniform regions with light surface winds.above flat, uniform regions with light surface winds.

U.S. air masses originate from:U.S. air masses originate from:a.a. maritime Polar (mP)maritime Polar (mP)b.b. continental Polar (cP)continental Polar (cP)c.c. maritime Tropical (mT)maritime Tropical (mT)d.d. continental Tropical (cT)continental Tropical (cT)

Letter 'k' may be added at the end--this indicates that the Letter 'k' may be added at the end--this indicates that the air mass is colder than the land below, which warms the air mass is colder than the land below, which warms the lower air and causes instability.lower air and causes instability.

cP Wind FlowcP Wind Flow

Figure 12.3Figure 12.3

Western Western mountains mountains normally protect normally protect the Pacific the Pacific Northwest from Northwest from cP air.cP air.

Strong highs, Strong highs, however, can however, can create northeast create northeast winds that cause winds that cause cold outbreaks cold outbreaks along the western along the western coast.coast.

Summer & Winter cP AirSummer & Winter cP AirSummer cP air over Summer cP air over the US brings the US brings welcome relief from welcome relief from heat, but also triggers heat, but also triggers steeper steeper environmental lapse environmental lapse rates and cumulus rates and cumulus cloud development.cloud development.

Cold surfaces during Cold surfaces during the winter create the winter create temperature temperature inversionsinversions..

Figure 12.4Figure 12.4

Modification of cP AirModification of cP AirFigure 12.5Figure 12.5

As the cP air mass moves over the warmer Gulf of As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream waters, surface warmed air Mexico and Gulf Stream waters, surface warmed air becomes unstable, rises, and forms extensive rows of becomes unstable, rises, and forms extensive rows of cumulus cloud streets.cumulus cloud streets.

Origin of mP AirOrigin of mP Air

Figure 12.6Figure 12.6

Cold Asian & Cold Asian & polar air passing polar air passing over the ocean over the ocean south of the south of the Aleutian low will Aleutian low will pick up warmth pick up warmth and moisture, and and moisture, and reaches the Pacific reaches the Pacific Coast as cool, Coast as cool, moist, and moist, and unstable, bringing unstable, bringing rain and snow.rain and snow.

Air Mass Movement

Modification of mP AirModification of mP AirFigure 12.7Figure 12.7

Orographic precipitation lowers the moisture content of mP air during Orographic precipitation lowers the moisture content of mP air during its westward flow.its westward flow.

Leeward of the Rockies, the air is dry. Chinook winds can form if Leeward of the Rockies, the air is dry. Chinook winds can form if conditions are favorable.conditions are favorable.

East Coast mP AirEast Coast mP Air

A strong anticyclone in A strong anticyclone in eastern Canada creates eastern Canada creates northeasterly winds that northeasterly winds that may bring cold, unstable may bring cold, unstable Atlantic mP air and Atlantic mP air and storms into New storms into New England and the middle England and the middle Atlantic States.Atlantic States.

These storms are known These storms are known as nor’easters.as nor’easters.

Figure 12.8Figure 12.8

Figure 12.9Figure 12.9

Warm and moist maritime air from the tropical Pacific may reach the Warm and moist maritime air from the tropical Pacific may reach the West Coast as a series of unstable waves, bringing powerful West Coast as a series of unstable waves, bringing powerful thunderstorms and heavy rainsthunderstorms and heavy rains..

“Pineapple Express”

Tropical Pacific Tropical Pacific mT AirmT Air

Gulf & Caribbean mT AirGulf & Caribbean mT Air

Gulf of Mexico and Gulf of Mexico and Caribbean Sea warmth Caribbean Sea warmth and moisture flows and moisture flows into the East Coast into the East Coast due to a strong due to a strong anticyclone.anticyclone.

When it rises above When it rises above dense cP air, heavy dense cP air, heavy and widespread and widespread precipitation can precipitation can result.result.

Figure 12.10Figure 12.10

cT AircT Air

Figure 12.11Figure 12.11

Dry, hot air from the Dry, hot air from the Mexican desert can Mexican desert can cause low level cause low level instability in the U.S. instability in the U.S. interior during interior during summer, and may summer, and may trigger dust devils.trigger dust devils.

An upper level ridge An upper level ridge of high pressure may of high pressure may add compressionally add compressionally heated air to the heated air to the region, enhancing the region, enhancing the dry, hot conditions.dry, hot conditions.

Air Masses and FrontsAir Masses and Fronts

-Most “weather” occurs in a narrow “battle zone” located between large -Most “weather” occurs in a narrow “battle zone” located between large air masses…terminology developed during WWIair masses…terminology developed during WWI-This transition zone is called a front…boundary of very strong -This transition zone is called a front…boundary of very strong differences in temperature and humiditydifferences in temperature and humidity-Four different fronts are used on weather maps:-Four different fronts are used on weather maps:

Figure 12.12Figure 12.12

Types of Fronts

• Cold front: cold, dry air replacing warmer moist air

• Warm front: warm, moist air replacing colder air

• Stationary front: boundary between air masses is not moving

• Occluded front: frontal boundary lifted above another frontal boundary

Front IdentificationFront Identification

Figure 12.13Figure 12.13

Locating a front on a Locating a front on a weather map involves weather map involves finding sharp changes in: finding sharp changes in: a) temperaturea) temperatureb) dew pointb) dew pointc) wind directionc) wind directiond) pressure and d) pressure and e) cloud/precipitation e) cloud/precipitation patterns.patterns.In this figure, pressure In this figure, pressure tendency is shown as the tendency is shown as the line to the right of the line to the right of the station plot station plot

Cold Front

Cold Front Animated

Two types of Cold Fronts

Katafront: Some cold fronts produce very little or no precipitation as they move across the country. The only sign a front has moved through your area is a sudden change in winds and temperature.

Anafront:A cold front breeds more precipitation when weak upper-level winds relative to stronger surface winds stretch the cold front out and the slope of the cold air becomes much less. This usually occurs when the upper-level winds become parallel to the front.

Cold Front TransitionCold Front TransitionFigure 12.14Figure 12.14

Important cloud, wind, and temperature changes are revealed in this Important cloud, wind, and temperature changes are revealed in this cross-section view of a typical cold front.cross-section view of a typical cold front.The front slopes steeply (1km in 50km), and cirriform clouds protrude The front slopes steeply (1km in 50km), and cirriform clouds protrude ahead.ahead.

Cold Front Clouds

Strengthening FrontStrengthening Front

Satellite imagery shows the transition between a weak front Satellite imagery shows the transition between a weak front and its frontogenesis, or strengthening, as it moves offshore and its frontogenesis, or strengthening, as it moves offshore over warmer water.over warmer water.

Figure 12.15AFigure 12.15AFigure 12.15BFigure 12.15B

Back Door Cold FrontBack Door Cold Front

Eastern Canadian high Eastern Canadian high pressure can generate cold pressure can generate cold fronts from the northeast, fronts from the northeast, which mix with the warm, which mix with the warm, moist Gulf air.moist Gulf air.

Cold air damming describes Cold air damming describes how the Appalachian how the Appalachian Mountains confine the Mountains confine the front's westward movement.front's westward movement.

Figure 12.16Figure 12.16

Warm FrontsWarm Fronts

Figure 12.17Figure 12.17

Observed wind, Observed wind, temperature, pressure, temperature, pressure, humidity, clouds, and humidity, clouds, and rain patterns experienced rain patterns experienced before, during, and after before, during, and after a front.a front.

Note the rotation of Note the rotation of winds and change in winds and change in temperature along this temperature along this warm front.warm front.

Warm Front TransitionWarm Front Transition

Figure 12.18AFigure 12.18A

Unique clouds and precipitation patterns are associated with Unique clouds and precipitation patterns are associated with warm frontswarm fronts --A broader range of precipitation than in a cold front.--A broader range of precipitation than in a cold front.The cross-sectional view shows the gentle slope of The cross-sectional view shows the gentle slope of overrunning warm air, a typical temperature inversion, and overrunning warm air, a typical temperature inversion, and the shifting windsthe shifting winds..

Warm Front Animation

• Less dense, warm air gradually climbs surface of cooler air

Warm Front

Warm Front Clouds

Occluded Fronts

• When a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called an occluded front or simply an occlusion

• Represented as a solid purple line with alternating cold front type triangles and warm front half circles

• Two types: Warm and cold occlusions– Cold occlusions most prevalent in the Pacific coastal states– Warm occlusions occur when the milder, lighter air behind a cold

front is unable to lift the colder heavier air off the ground and instead rides up along the sloping warm front

Occluded FrontsOccluded FrontsFast moving cold front may Fast moving cold front may overtake the slower moving warm overtake the slower moving warm front, particularly when they are front, particularly when they are influenced by cyclonic winds.influenced by cyclonic winds.

Cold occlusionCold occlusion describes this describes this scenario with very cold air scenario with very cold air overtaking the warm sectorovertaking the warm sector

Stationary Front• A front with essentially no movement• Drawn as alternating red and blue line.

Semicircles face toward colder air on the red line and triangles point toward warmer air on the blue line.

• Winds tend to blow parallel to a stationary front.

• If either a cold or warm front stops moving, it becomes a stationary front

Stationary Front—Parallel Upper Level Winds

Fronts, Jets, and Cyclones• Fronts are associated with mid latitude

cyclones--which are caused by jet streams.

Mid-Latitude CycloneMid-Latitude Cyclone

Occluded fronts are Occluded fronts are common along mid-latitude common along mid-latitude cyclones, or deep low cyclones, or deep low pressures centers about pressures centers about which the cold and warm which the cold and warm fronts pivot.fronts pivot.

These storms appear These storms appear frequently in the mid-frequently in the mid-latitudeslatitudes..

Figure 12.21Figure 12.21

Upper Air FrontUpper Air Front

Figure 12.22Figure 12.22

A division between A division between cold and warm air cold and warm air masses in the masses in the tropopause is tropopause is described as an described as an upper-air front, upper-air front, which forms when which forms when polar jet rides above polar jet rides above the tropopause the tropopause through tightly through tightly packed isotherms.packed isotherms.

Midlatitude Life Cycle (Continued in next chapter)

Key Facts and Concepts• An air mass is a large body of air with similar horizontal

temperature and moisture characteristics• Regions where air masses originate and acquire their

properties of temp and moisture are called source regions. These regions are generally flat, of uniform composition, where winds are light

• cP are cold and dry; cA extremely cold and dry; cT hot and dry; mT warm and moist; and mP cool/cold and moist

• A front is a sharp transition zone that separates two air masses with contrasting meteorological properties

• A cold front is a region where cold air is replacing warmer air. Along the leading edge of the cold front, warm air is forced rapidly upward, producing a narrow band of showery precipitation

Con’t• The cold front represents a trof of lower pressure

where there is a wind shift (most often from SW to NW). After the passage of a cold front, temperatures drop and pressures rise

• A warm front is a region where warmer air is replacing colder air. At the warm front, the warm air slowly rises up and over the colder air—producing stratiform clouds and continuous precip ahead of the warm front

• As the warm front approaches, the clouds often change from high clouds (Ci and Cs), to middle clouds (As) to low clouds (Ns and St)

• An occluded front often has the characteristics of both a cold front and a warm front