AOSS 102: Extreme Air Masses & Fronts 4 November 2015

Air Mass Classifications

Definion: Extremely large body of air whose properes of temperature and humidity are fairly similar in any horizontal direcon at any given altude. Generally flat areas with light winds, dominated by surface high pressure.

Table 9.1, p. 239 Air Mass Classifications

Air masses obtain their characteriscs from the source region over which the form. Regions dominated by persistent areas of high pressure are good source regions. Fig. 9.2, p. 240 Air Mass Classifications

It is the connental arcc (cA) air mass that produce the extreme, cold as they move across North America.

Table 9.1, p. 239 Connental Polar (cP) and Connental Arcc (cA) Air Masses

Fig. 9.3, p. 240

Winter Continental Polar air typically has a temperature less than 0°C. Continental Arctic air typically has a temperature less than - 20°C.

The leading edge of an air mass at the surface is defined by a “front”. A front is a “transition zone” between two distinct air masses. Extremely Cold Outbreak: Surface Weather Map

snow

12/24/1983

NOTE: The Rocky Mountains and Appalachian Mountains

often protect the coasts from the most extreme cold. Fig. 9.5, p. 243 Extremely Cold Outbreaks: Typical Upper-Level Wind Flows

12/1990 12/1989

Observed minimum temperatures during each cold snap (°F) Position of the surface high

$480 million in damage to fruit and vegetable crops in Florida and Texas

Fig. 9.4, p. 241 Modificaon of Cold Air Masses: cP warms and moistens over ocean waters

Warm and moist air destabilizes, forms cumulus streets and showers, cP assumes qualities of maritime air Fig. 9.6, p. 244 Air Mass Classification

Marime polar (mP) air masses are responsible for the cold, damp and oen wet weather along the northeast coast of North America, as well as for the cool, rainy winter weather along the west coast of North America.

Table 9.1, p. 239 Marime polar air originates over Asia as polar and arc air masses, is modified (warms, moistens) as it moves over ocean

Aleutian low quasi-stationary Cumulus in January over the ocean, (rain sign of unstable and snow) on land, air triggered by mountains that force the moist cold air upwards

Fig. 9.7, p. 245 Marime Polar air masses are also oen modified due to repeated transport over western U.S. mountain ranges: becomes dry Pacific Air (fair weather, cool)

Fig. 9.8, p. 245 Atlanc mP air masses are typically colder than their Pacific counterparts.

Typical late winter or early paern that carries cold, moist air to New England states. Staonary frontal zone oen develops.

Fig. 9.9, p. 246 Air Mass Classification

Marime tropical (mT) air masses are responsible for hot, muggy weather that frequently plagues the eastern half of the U.S. in .

Table 9.1, p. 239 Pineapple Express Heavy rain and often extensive flooding in northern and central

Origin: subtropical east Pacific Ocean, air travels over 1000 miles of water before reaching the coast Fig. 9.10, p. 247 Pineapple Express: Flooding in Yosemite National Park

January 1997 Fig. 9.11, p. 247 Warm Maritime Tropical Air in Eastern/Central U.S.

Upper level Surface low

Unseasonably hot spell in the eastern poron of the U.S.

Upper level low and high

April/17/1976 Fig. 9.12, p. 248 Air Mass Classification

Connental tropical (cT) air masses only form in the summer in the U.S. Responsible for summer heat waves in the western half of the U.S.

Table 9.1, p. 239 Connental Tropical (cT) Air Masses Source region: northern Mexico and adjacent arid southwestern U.S.

Dry and hot weather, heat records were broken in southwestern US, Las Vegas 7 consecuve Posion of an Death Valley days of heat in upper-level High, 7/2005. sinking moon, July 2005 Fig. 9.14, p. 249 very weak winds cT Air Masses: Extreme Heat in June 1990

Phoenix suspended aircra operaons when temperature reached 122 °F: so asphalt, air density too low to li planes

June 26, 1990 p. 250 Image Quiz Image Quiz What is a front?

• A sloping transion zone between two air masses of different density.

– Since the temperature distribuon is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature.

– Fronts also separate air masses of differing moisture content. Four major types of fronts

• STATIONARY FRONT (no movement) • COLD FRONT • WARM FRONT • OCCLUDED FRONT Fig. 9.16, p. 252 Cold Front: Vercal Extent

Fig. 9.15, p. 251 Locate the Cold Front: large temperature and moisture (dew point) contrasts, shis in wind direcon, isobars kink as they cross front, pressure drops ahead of the front, rain

rain

Three hour pressure change

See Appendix C

Fig. 9.18, p. 253 Vercal View of the Weather Across a Cold Front

sky clears

Fig. 9.20, p. 254

At the front, cold, dense air floats under warm air, forcing the warm air upwards. Typically vapor condenses, forms clouds and precipitaon, are possible. Speed of leading edge: 10-25 mph Doppler radar image showing precipitaon paerns along a cold front

light rain

Thunderstorms

Steep slope of cold front can result in liing of warm, unstable air to form thunderstorms (red areas). Fig. 9.19, p. 254 Frontogenesis: Strengthening of a cold front seen in infrared satellite images

As front moves over warm Gulf stream, convecon develops and front intensifies again

Fig. 9.21, p. 255