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Chapter 9 : Air Mass • Air masses Contain uniform temperature and humidity characteristics.

• Fronts Boundaries between unlike air • Air Masses masses. • Fronts

• Fronts on Weather Maps ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Air Masses • Air masses have fairly uniform temperature and moisture Source Regions content in horizontal direction (but not uniform in vertical).

• Air masses are characterized by their temperature and humidity • The areas of the globe where air masses from are properties. called source regions. • The properties of air masses are determined by the underlying • A source region must have certain temperature surface properties where they originate. and humidity properties that can remain fixed for a • Once formed, air masses migrate within the general circulation. substantial length of time to affect air masses • UidilidliliUpon movement, air masses displace residual air over locations above it. thus changing temperature and humidity characteristics. • Air mass source regions occur only in the high or • Further, the air masses themselves moderate from surface low latitudes; middle latitudes are too variable. influences.

ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Cold Air Masses Warm Air Masses

January July January July

• The cent ers of cold ai r masses are associ at ed with hi gh pressure on surf ace weath er • The cent ers of very warm ai r masses appear as semi -permanentit regions of fl low maps. pressure on surface weather maps. • In summer, when the oceans are cooler than the landmasses, large high-pressure • In summer, low-pressure areas appear over desert areas such as American centers appear over North Atlantic (Bermuda high) and Pacific (Pacific high). Southwest, Southeast Asia, Central Africa, and near the equator. • In winter, high-pressure centers form over the northern parts of Asian continent • In winter, the semi-permanent low-pressure appear over the northern Pacific () and North America. ESS124 () and northern Atlantic (Icelandic Low). ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Classification of Air Masses Five Types of Air Masses

• Air masses are classified according to the temperature • Theoretically, there should be 6 and moisture characteristics of their source regions. types of air masses (2 moisture t3ttt)types x 3 temperature types). • Bases on moisture content: continental (dry) and • But mA-type (maritime Arctic) maritime (moist) does not exist. • Based on temperature: tropical (warm), polar (cold), • cA: continental Arctic arctic (extremely cold). cP: continental Polar • Naming convention for air masses: A small letter (c, cT: continental Tropical m) indicates the moist content followed by a capital letter (T, P, A) to represent temperature. mP: maritime Polar mT: maritime Tropical

ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Continental Polar ((cPcP)) Air Mass Modification of cPAir Masses

• Continental Polar air masses form over large, high- • Migrations of cP air induce latitude land masses, such as northern Canada or colder, drier conditions over Siberi a. affect ed areas. • cP air masses are cold and extremely dry. • As cP air migrates toward lower • Wintertime cooling over these land areas cause the latitudes, it warms from beneath. atmosphere to become very stable (even inversion). • As it warms, moisture capacity • The combination of dry and stable conditions ensure increases while stability that few if any clouds form over a cP source region. decreases. • Summer cP air masses are similar to winter cP, but much less extreme and remain at higher latitudes.

ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Continental Arctic ((cAcA)) Air Masses Continental Tropical ((cTcT)) Air Masses

• Mainly a summertime phenomenon exclusive to the desert southwest of the U.S. and northern Mexico. • Characteristically hot and very dry. • Very unstable, yet clear conditions • Continental Arctic (cA) air represents extremely cold and dry predominate due to a lack of water conditions as, due to its temperature, it contains very little water vapor. vapor. • Thunderstorms may occur when • The boundary between cA and cP air is the shallow (~1-2 km) arctic front. moisture advection occurs or when air is forced orographically. • cA air masses can extend as far southward as the Canadian- United State. ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Maritime Polar ((mPmP)) Air Masses Maritime Tropical ((mTmT)) Air Masses

• Maritime polar air masses form over • Form over low latitude oceans and as upper latitude oceanic regions and are such are very warm, humid, and cool and moist. unstable. • mP air masses form over high-latitude ocean as cP air masses move out from the • mT air masses from Atlantic and Gulf interior of continents. (i.e., cP Æ mP). of Mexico is the primary source • Oceans add heat and moisture into the dry region for the eastern U.S. and cold cP air masses. • As air advects over the warm • Along the west coast of the U.S., mP air continent in summer the high affects regions during winter and may be humidity and high heat occasionally present before mid-latitude combine to dangerous levels. advect over the continent. • mT air masses have an enormous • Along the east coast, mP air typically influence on the southwestern U.S, affects regions after passage as particularly in summer. the mP air wraps around the area of low ‰Advection of mT air also pressure. promotes the so-called Arizona ESS124 ESS124 Prof. Jin-Jin-YiYi Yu monsoon. Prof. Jin-Jin-YiYi Yu

Fronts Cold Fronts

• Meteorologists classify fronts • Cold fronts form when cold air based on the thermal and moisture displaces warm air. characteristics of the air masses, the direction of the movement of • Indicative of heavy precipitation the air masses, and whether the events, rainfall or snow, combined boundary between the airmasses is with rapid temperature drops. in contact with the ground (a surface front), or can be found • Steep front slope, typically 1:100. aloft (an upper level front). • Moving faster, up to 50 km/hr (30 • There are four general types of mph). fronts associated with mid-latitude • Northwesterly winds behind a cold cyclones with the name reflective of the advancing air mass. front, and southwesterly in ahead of the front.

ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Cold Front Weather • The type of precipitation that will occur Radar/Satellite Views of Cold Fronts along a cold front depends on the characteristics of the warm air ahead of the front. Radar Pi ctur e Satellite Picture • If the warm air is moist and conditionally unstable, thunderstorms can be triggered ahead of the cold front; the thunderstorms will form in a line called squall line; if thunderstorms form, tornadoes, hail, diiddamaging wind can occur. • If the warm air is stable, the clouds that form by lifting only produce light rain. • If the warm air is dry and stable, no clouds will form at all. ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Warm Fronts Warm Front Weather • Clouds are deepest just north of the warm front boundary and progressively becomes thinner and higher toward the north. • The intensity of the precipitation depends on the stability of the warm air. • If the warm air is conditionally unstable, thunderstorms may develop • Created when warm air displaces colder air. over the warm front. • Sha llow hor izon ta l s tra tus c lou ds an d lig ht prec ip itati on. • If the warm air is stable, the clouds will • Frontal fogs may occur as falling raindrops evaporate in be layered. the colder air near the surface. Sleet and freezing rain may • If the temperature in the cold air is also formed. below freezing, snow, ice pellets, or • Half the slope of cold fronts, typically (1:200). freezing rain may occur. ESS124 ESS124 • Moving slower, about 20 km/hr (12 mph). Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Stationary Fronts Radar/Satellite Views of Warm Fronts • When two unlike air masses remain side by side, with neither encroaching upon the other, a stationary front exists. Radar Pi ctur e Satellite Picture • The cold ai r on th e north sid e of th e f ront i s moving parallel to the front, while the warm air to the south moving toward the front and get lifted along the frontal boundary. • If the warm air is conditionally unstable, a line of showers and thunderstorms may develop in the warm side of the front. • If the warm air is stable, widespread layered clouds may form over the front, with rain falling on the cold side of the

ESS124 front. ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Occluded Fronts WarmWarm--and Cold-Cold-TypeType Occlusion Fronts • Occlusion: the warm air is cut off from the surface by the meeting of two fronts. warm-type cold-type • Usually, a fast-moving cold front catches a slow-moving warm front. •A cold-type occlusion: eastern half of the continent where a cold front associated with cP air meets a warm front with mP air ahead. •A warm-type occlusion: western edges of continents where the cold front, associated with mP air, invades an area in which colder cP air is entrenched.

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Dry Line UpperUpper--LevelLevel Front • In the south-central US (Texas, • Air masses do not necessary extend Oklahoma, Kansas region) and to the surface of the Earth. northern Mexico, a dry lines can • Air masses can be “stacked” in the developpy between the dry air vertical , so th at an ai r mass flowing eastward from the high boundary (i.e., front) can be desert plateau regions of present aloft. Arizona, Colorado, New Mexico and Mexico encounters the • This type of front is called “cold moist air flowing northward front aloft” or “upper-level front”. from the Gulf of Mexico. • Upper-level fronts are common • The moi st ai r will ri se over th e feature within cyclones that form dry air. east of the Rockies. • If the moist air is conditionally unstable, thunderstorm will • Upper-level fronts develop when the air from the west of the trough (which develop along the dry line. goes through adiabatically descending) meets the air from the east of the trough ESS124 (which goes through moist adiabatically ascending). ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

Developments of Low-Low- and HighHigh--PressurePressure Centers Finding Fronts on Weather Maps • Dynamic Effects: Combined curvature Not a boundary and jetstreak effects produce upper-level between air masses, convergence on the west side of the but boundary between trough to the north of the jetsreak, which air coming from add air mass into the vertical air column Canada and the anddd tend to prod uce a surf fhihace high- Rockies. pressure center. The same combined Î Cold air damming. effects produce a upper-level divergence on the east side of the trough and favors the formation of a low-level low-pressure center. • Thermodynamic Effect: heating Î surface low pressure; cooling Î surface high pressure. • Frictional Effect: Surface friction will cause convergence into the surface low-pressure • A warm, cold, and stationary front will be located on the warm edge of a shape center after it is produced by upper-level dynamic effects, which adds air mass into the low temperature gradient. center to “fill” and weaken the low center (increase the pressure) • Lower dewpoint temperatures can sometime be found on the cold air side of a cold, • Low Pressure: The evolution of a low center depends on the relative strengths of the upper- warm, or stationary front. level development and low-level friction damping. • High Pressure: The development of a high center is controlled more by the convergence of • If the wind on the cold side blows toward the front, a cold front; away from the front, surface cooling than by the upper-level dynamic effects. Surface friction again tends to a warm front; along the front, a stationary front; sharp wind shift, a occluded front. destroy the surface high center. ESS124 ESS124 Prof. Jin-Jin-YiYi Yu Prof. Jin-Jin-YiYi Yu

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Figure 9A

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