AOSS 102: Extreme Weather Air Masses & Fronts 4 November 2015
Air Mass Classifications
Defini on: Extremely large body of air whose proper es of temperature and humidity are fairly similar in any horizontal direc on at any given al tude. Generally flat areas with light winds, dominated by surface high pressure.
Table 9.1, p. 239 Air Mass Classifications
Air masses obtain their characteris cs 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 con nental arc c (cA) air mass that produce the extreme, cold winters as they move across North America.
Table 9.1, p. 239 Con nental Polar (cP) and Con nental Arc c (cA) Air Masses
Fig. 9.3, p. 240
Winter Continental Polar air typically has a temperature less than 0°C. Winter 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 Modifica on of Cold Air Masses: cP warms and moistens over ocean waters
Warm and moist air destabilizes, forms cumulus cloud streets and rain showers, cP assumes qualities of maritime air Fig. 9.6, p. 244 Air Mass Classification
Mari me polar (mP) air masses are responsible for the cold, damp and o en 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 Mari me polar air originates over Asia as polar and ar c air masses, is modified (warms, moistens) as it moves over ocean
Aleutian low quasi-stationary Cumulus clouds in January over the ocean, Precipitation (rain sign of unstable and snow) on land, air triggered by mountains that force the moist cold air upwards
Fig. 9.7, p. 245 Mari me Polar air masses are also o en modified due to repeated transport over western U.S. mountain ranges: becomes dry Pacific Air (fair weather, cool)
Fig. 9.8, p. 245 Atlan c mP air masses are typically colder than their Pacific counterparts.
Typical late winter or early spring pa ern that carries cold, moist air to New England states. Sta onary frontal zone o en develops.
Fig. 9.9, p. 246 Air Mass Classification
Mari me tropical (mT) air masses are responsible for hot, muggy weather that frequently plagues the eastern half of the U.S. in summer.
Table 9.1, p. 239 Pineapple Express Heavy rain and often extensive flooding in northern and central California
Origin: subtropical east Pacific Ocean, air travels over 1000 miles of water before reaching the southern California 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 jet stream Surface low
Unseasonably hot spell in the eastern por on of the U.S.
Upper level low and high
April/17/1976 Fig. 9.12, p. 248 Air Mass Classification
Con nental 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 Con nental 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 consecu ve Posi on of an Death Valley days of heat in upper-level High, 7/2005. sinking mo on, July 2005 Fig. 9.14, p. 249 very weak winds cT Air Masses: Extreme Heat in June 1990
Phoenix suspended aircra opera ons 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 transi on zone between two air masses of different density.
– Since the temperature distribu on 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: Ver cal Extent
Fig. 9.15, p. 251 Locate the Cold Front: large temperature and moisture (dew point) contrasts, shi s in wind direc on, 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 Ver cal 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 precipita on, thunderstorms are possible. Speed of leading edge: 10-25 mph Doppler radar image showing precipita on pa erns along a cold front
light rain
Thunderstorms
Steep slope of cold front can result in li ing 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, convec on develops and front intensifies again
Fig. 9.21, p. 255