Lecture #14 March 29, 2010, Monday Air Masses & Fronts General definitions air masses source regions fronts Air masses formation types Fronts formation types
Air Masses General Definitions a large body of air with fairly uniform temperature and humidity characteristics continental in size uniform horizontally, not vertically Source Regions where air masses form and acquire their characteristics land or ocean high or low latitude Fronts boundaries between air masses of unlike temperature and/or humidity characteristics spatially limited zones rather than a line. inherently linked to mid-latitude cyclones
Formation of Air Masses Surface energy & moisture exchanges decide pressure, temperature, moisture characteristics of overlying air. Such exchanges vary from place to place and lead to variation of above parameters of the air mass. Air masses form when air stagnates over particularly large surface regions, typically those that are topographically uniform. Overlying air gains temperature and humidity characteristics of the surface. Air masses are characterized based on these properties. Maritime air = wet, continental air = dry From warmest to coldest: tropical, polar, arctic Once formed, air masses migrate within general circulation.
North American air masses and air mass source regions
Source regions of global air masses
Moisture characteristics are expressed first and in lower case in the classification system. Temperature is expressed next in upper case. Continental Polar (cp) or Continental Arctic (ca or A) dry, cold Maritime Polar (mp) moderately moist, moderate temperatures (cool) Maritime Tropical (mt) very moist, warm Continental Tropical (ct) dry, hot Types of Air Masses
Continental Polar (cp) & Continental Arctic (ca) Air Masses Wintertime source regions for cp: northern Canada and Asia. cp: cold and dry, inherently stable. During summer, cp is warmer and more humid by comparison, but still cool and dry compared to other air masses. ca: extremely cold and dry due to its temperature, it contains very little water vapor. Arctic front: the boundary between ca and cp air masses. cp mid-latitude (~60 o N) greater vertical extent than arctic front ca ct or mt
Maritime Polar (mp) Air Masses Form over upper latitude oceanic regions, cool and moist West coast of the US, mp affects regions during winter, may be present before mid-latitude cyclones advect over the continent. East coast of US, 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: 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, very warm and humid mt: inherently unstable due to high temperature & humidity. The Gulf of Mexico is the primary source region of mt for eastern U.S. As air advects over the warm continent in the summer, the high humidity and high heat occasionally combine to dangerous levels (heat index). Advection of mt air masses to eastern California and Arizona also promotes the so-called Arizona monsoon (rainy season).
Modification of cp air masses when they move to the South
Lake effect snows intensify with cp air mass
Lake effect snows intensify with cp air mass
Lake Effect Snow
Fronts: General Characteristics All fronts separate air masses of different densities. All fronts slope over cold (dense) air. Warm air is less dense than cold air. Moist air is less dense than dry air.
Type of Fronts Cold Front cold air mass invades and displaces warm air mass Warm Front warm air mass invades and displaces cold air mass Stationary Front no movement of frontal boundary similar in structure as cold front Occluded Front boundary between two cold air masses, as one cold front overrunning a warm front ahead of it
Weather map symbols: 1. cold front; 2. warm front; 3. stationary front; 4. occluded front; 5. surface trough; 6. squall/shear line; 7. dry line; 8. tropical wave
Cold Front cumulus, cumulonimbus short but intense precipitation limited horizontal extent, fast moving Cold cp Warm mt steep frontal slope (1:100) strong uplifting (convergence) Above: the vertical displacement of warm air along a cold front boundary. Right: the sharp cold front boundary is evident on both satellite pictures and radar composites.
Warm Front stratus low intensity precipitation low frontal slope (1:200) gentle uplift Warm air mass overrunning cold air mass, large amounts of warm, moist air climbs over cooler, drier air along horizontally extensive spatial areas. Stable (inversion) air above the warm front limits vertical extent and forms wide band clouds and light precipitation In extreme situations, sleet & freezing rain may result.
Vertical wind shear (wind speed increases with altitude due to friction decreasing with altitude) leads to the slope of warm from decreases with time.
Stationary Front Cold cp Stationary Front Warm mt When two unlike air masses remain side by side, with neither encroaching upon the other, a stationary front forms. Stationary fronts may slowly migrate and warmer air is displaced above colder air, with frontal surface inclined, sloping over cold air Because fronts are zones of transition rather than sharp (line) boundaries hard to detect small movement of this type of front.
cp mp Occlusion sequence mp cp Occluded Front cp mp
Occluded Fronts When cold and warm 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 vertical 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.
cp mp Occluded Fronts mp cp
Occluded Fronts 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
Humidity is an important determinant of air density, air masses with similar temperatures but strong humidity difference will have different density, and the boundaries between dry and moister airmasses with similar temperatures are called drylines They frequently occur throughout the Great Plains between ct & mt airmasses and are an important contributors to storm development Dry lines ct mt A dryline over Texas
mt ct Dryline: sharp drop in humidity without appreciable drop in temperature. ct is more dense than mt.