Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt Anthony J. Vega

Part 3. Distribution and Movement of Air Chapter 9 Air Masses and Fronts

Introduction Air masses contain uniform temperature and humidity characteristics They affect vast areas Fronts are boundaries between unlike air masses Fronts are spatially limited They are inherently linked to mid-latitude cyclones

Formation of Air Masses Surface energy and moisture exchanges cause initial temperature, pressure, and moisture characteristics in overlying air Such exchanges are limited in spatial scope, leading to variation in these parameters from place to place Source regions Sites of air mass genesis Form when air stagnates over particularly large surface regions, typically those which are topographically uniform Overlying air gains temperature and humidity characteristics of the surface Air masses are characterized based on these properties Moisture characteristics are expressed first and in lower case in the classification system Temperature is expressed next in upper case

The wettest air is called maritime air, while the driest is called continental Air deemed, from warmest to coolest, tropical, polar, or arctic Once formed, air masses migrate within the general circulation Upon movement, air masses displace residual air over locations thus changing temperature and humidity characteristics Further, the air masses themselves moderate from surface influences North American air masses and air mass source regions

Continental Polar (cP) and Continental Arctic (cA) Air Masses Wintertime source regions for continental polar (cP) air include northern Canada and Asia cP air takes on cold, dry characteristics and is inherently stable During summer, cP air is warmer and more humid by comparison but still cool and dry as compared to other air masses Continental Arctic (cA) air represents extremely cold and dry conditions as, due to its temperature, it contains very little water vapor The boundary between cA and cP air is the arctic front

Migrations of cP air induce colder, drier conditions over affected areas. As cP air migrates toward lower latitudes it warms from beneath. As it warms, moisture capacity increases while stability decreases. This sequence depicts the movement of cP air equatorward.

Maritime Polar (mP) Air Masses Maritime polar air masses form over upper latitude oceanic regions and are cool and moist Along the west coast of the U.S., mP air affects regions during winter and may be present before mid-latitude cyclones advect over the continent Along the east coast, mP air typically affects regions after cyclone passage as the mP air wraps around the area of low pressure –Referred to as a Nor’easter for the dominant northeasterly winds Continental Tropical (cT) 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 predominate due to a lack of water vapor Thunderstorms may occur when moisture advection occurs or when air is forced orographically

Maritime Tropical (mT) Air Masses Form over low latitude oceans and as such are very warm and humid mT air is inherently unstable due to high temperature and humidity The Gulf of Mexico is the primary source region for the eastern U.S. As air advects over the warm continent in summer the high humidity and high heat occasionally combine to dangerous levels Advection of mT air also promotes the so-called Arizona monsoon Fronts Fronts separate air masses and bring about changes in temperature and humidity as one air mass is replaced by another There are four general types of fronts associated with mid-latitude cyclones with the name reflective of the advancing air mass

The four fronts

Cold Fronts –Cold fronts form when cold air displaces warm air –Indicative of heavy precipitation events, rainfall or snow, combined with rapid temperature drops –Extreme precipitation stems from rapid vertical lifting associated with the steep cold front boundary profile –Because cold air is dense, it spills across the surface producing a steeply inclined leading edge –Warm moist air ahead of the front is forced aloft with great vertical displacement –This accounts for large vertical cumulonimbus clouds and heavy precipitation –Such sharp transitions between the colder, drier air behind the front and the warmer, moisture air ahead of the front, can be easily detected on satellite images and radar composites Frontal development about a low pressure system

Above: The vertical displacement of air along a cold front boundary Right: The sharp cold front boundary is evident on both satellite pictures and radar composites

Warm Fronts –Created when warm air displaces colder air –Even though the warmer air advances, it is displaced aloft –This overrunning process places large amounts of warm, moist air over cooler, drier air along extensive spatial areas –Shallow horizontal stratus clouds dominate and bring light precipitation to affected regions –Stable regions above the warmer air aloft help propagate vertically limited clouds and light precipitation –Frontal fogs may occur as falling raindrops evaporate in the colder air near the surface –In a similar, but more extreme situation, sleet and freezing rain may result Stationary Fronts –When two unlike air masses remain side by side, with neither encroaching upon the other, a stationary front exists –Fronts may slowly migrate and warmer air is displaced above colder –Fronts are zones of transition rather than sharp boundaries

Profile of a warm front

Occluded Fronts When two fronts meet, the warm air mass between them is displaced aloft resulting in an occluded front This typically occurs when a cold front closes on, and meets a warm front as it circulates about the low pressure center of a mid-latitude cyclone Cold air now occupies the surface completely around the low while warmer air is displaced aloft A cold-type occlusion usually occurs in the eastern half of the continent where a cold front associated with cP air meets a warm front with mP air ahead –This situation resembles a cold front in profile A warm-type occlusion is typical of the western edges of continents where the cold front, associated with mP air, invades an area in which colder cP air is entrenched –This results in a vertical profile which resembles a warm front

Occlusion sequence

Some occlusions form when the surface low elongates and moves away from the junction of the cold and warm fronts Some occlusions occur when the intersection of the cold and warm fronts slides along the warm front

Drylines –Because humidity is an important determinant of air density, air masses with similar temperatures but strong humidity gradients will act as fronts –Boundaries between dry and moister air are called drylines –They frequently occur throughout the Great Plains and are an important contributor to storm development A dryline over Texas

End of Chapter 9 Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt