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Unformatted text preview: Air Masses & Fronts Air This chapter discusses: 1. Classification of 4 North American air masses based Classification on cold or warm land mass and ocean origin on 1. Air mass delineation and movement along stationary, Air cold, warm, and occluded fronts cold, Regional Weather Patterns Regional
Surface maps of Surface US temperature, dew point, and pressure reflect synoptic trends. synoptic In this image, In nearly every station around the high pressure anticyclone reports cold, dry air, suggesting the air mass formed in a common region. common Figure 12.1 Source Regions & Classification Source Figure 12.2 Air masses of similar temperature and humidity form above flat, uniform regions with light surface winds. above U.S. air masses originate from: U.S. a. maritime Polar b. continental Polar c. maritime Tropical d. continental Tropical Letter 'k' is added to indicate the air mass is colder than the land below, which warms the lower air and causes instability. causes cP Wind Flow cP
Western Western mountains, such as the Rocky and Sierra Nevada ranges, normally protect the Pacific Northwest from cP air. cP Strong Strong anticyclone highs, however, can create northeast winds that cause cold outbreaks along the western coast. coast. Figure 12.3 Summer & Winter cP Air Summer Summer cP air over Summer the US brings welcome relief from heat, but also triggers steeper environmental lapse rates and cumulus cloud development. cloud Cold surfaces during Cold the winter create temperature inversions. inversions.
Figure 12.4 Modification of cP Air Modification Figure 12.5 As the cP air mass moves over the warmer Gulf of Mexico and Gulf As Stream waters, surface warmed air becomes unstable, rises, and forms extensive rows of cumulus cloud streets. forms Origin of mP Air Origin
Cold Asian & Cold polar air passing over the ocean south of the Aleutian low will pick up warmth and moisture, and reaches the Pacific Coast as cool, moist, and unstable, bringing rain and snow. and Figure 12.6 Modification of mP Air Modification Figure 12.7 Orographic precipitation lowers the moisture content of mP air, Orographic called Pacific air east of the Rockies, during its westward flow. called Leeward of the Rockies, the air is dry and may develop chinook Leeward winds. winds. East Coast mP Air East A strong anticyclone in strong eastern Canada creates northeasterly winds that may bring cold, unstable Atlantic mP air and storms into New England and the middle Atlantic States. middle These storms are known as These northeasters. northeasters. Figure 12.8 Tropical Pacific mT Air Tropical Figure 12.9 Warm and moist maritime air from the tropical Pacific may reach Warm the West Coast as a series of unstable and powerful thunderstorms. the Gulf & Caribbean mT Air Gulf Figure 12.10 Gulf of Mexico and Caribbean Sea warmth and moisture flows into Gulf the East Coast by a strong anticyclone. the When it rises above dense cP air, heavy and widespread When precipitation can result. precipitation Mexican Origion of cT Air Mexican
Dry, hot air from Dry, the Mexican desert can cause low level instability in the U.S. interior during summer, and may trigger dust devils. trigger An upper level ridge An of high pressure may add compressionally heated air to the region, enhancing the dry, hot conditions. conditions. Figure 12.11 Air Mass Fronts Air Figure 12.12 Two air masses entering a region, such as the U.S. middle latitudes, Two have a front, or transition zone, between the strong temperature and humidity differences. and Four different fronts are used on weather maps. Front Identification Front
Locating a front on Locating a weather map involves finding sharp changes in a) temperature a) b) dew point c) wind direction d) pressure and e) d) cloud/precipitation patterns. patterns. In this figure, In pressure tendency is shown as the line to the right of the station, where there is rising pressure. pressure. Figure 12.13 Cold Front Transition Cold Figure 12.14 Important cloud, wind, and temperature changes are revealed in Important this cross-section view of a typical cold front. this The front rises steeply (1km in 50km), and cirriform clouds The protrude ahead. protrude Frontogenesis describes a strengthening system, which may send a Frontogenesis fast moving squall line of showers ahead of the front. fast Strengthening Front Strengthening Figure 12.15A Figure 12.15B Satellite imagery shows the temporal transition between a weak Satellite front and its frontogenesis, or strengthening, as it moves offshore over warmer water. over Back Door Cold Front Back Eastern Canadian high Eastern pressure can generate cold fronts from the northeast, which mix with the warm, moist Gulf air. moist Cold air damming describes Cold how the Appalachian Mountains confine the front's westward movement. movement.
Figure 12.17 Frontal Weather Trends Frontal Observed wind, Observed temperature, pressure, humidity, clouds, and rain weather experiences typical patterns before, during, and after a front. after Note the rotation of winds Note and change in temperature along this warm front. along Figure 12.17 Warm Front Transition Warm Figure 12.18A Unique clouds and precipitation patterns are associated with warm Unique fronts, with a broader range of showers than in a cold front. fronts, The cross-sectional view shows the gentle slope of overrunning The warm air, a typical temperature inversion, and the shifting winds. warm Occluded Fronts Occluded Figure 12.19A Figure 12.20A Fast moving cold fronts may overtake the slower moving Fast warm front, particularly when they are influenced by cyclonic winds. cyclonic Cold occlusion describes this scenario with very cold air, as Cold compared with the warm occlusion. compared Mid-Latitude Cyclone Mid-Latitude Occluded fronts are common Occluded along mid-latitude cyclones, or deep low pressures centers about which the cold and warm fronts pivot. warm These storms appear These frequently in the midfrequently latitudes. Figure 12.21 Upper Air Front Upper
A division division between cold and warm air masses in the tropopause is described as an upper-air front, which forms when polar jet rides above the tropopause through tightly packed isotherms. isotherms. Figure 12.22 ...
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This note was uploaded on 03/15/2010 for the course ATOC ATOC 250 taught by Professor Gyakum during the Spring '09 term at McGill.
- Spring '09