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WX-201 Chapter 12 Air Masses and Fronts

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1 WX-201 Chapter 12 Air Masses and Fronts

2 Air Masses & Fronts This chapter discusses:
Classification of 4 North American air masses based on cold or warm land mass and ocean origin Air mass delineation and movement along stationary, cold, warm, and occluded fronts

3 Regional Weather Patterns
Surface maps of US temperature, dew point, and pressure reflect synoptic trends. In this image, nearly every station around the high pressure anticyclone reports cold, dry air, suggesting the air mass formed in a common region. Figure 12.1

4 Source Regions for Air Masses

5 US Source Regions & Classification
Figure 12.2

6 Air masses of similar temperature and humidity form above flat, uniform regions with light surface winds. U.S. air masses originate from: maritime Polar (mP) continental Polar (cP) maritime Tropical (mT) continental Tropical (cT) Letter 'k' may be added at the end--this indicates that the air mass is colder than the land below, which warms the lower air and causes instability.

7 cP Wind Flow Western mountains normally protect the Pacific Northwest from cP air. Strong highs, however, can create northeast winds that cause cold outbreaks along the western coast. Figure 12.3

8 Summer & Winter cP Air Figure 12.4 Summer cP air over the US brings welcome relief from heat, but also triggers steeper environmental lapse rates and cumulus cloud development. Cold surfaces during the winter create temperature inversions.

9 Modification of cP Air Figure 12.5 As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream waters, surface warmed air becomes unstable, rises, and forms extensive rows of cumulus cloud streets.

10 Origin of mP Air Cold Asian & 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. Figure 12.6

11 Air Mass Movement

12 Modification of mP Air Figure 12.7 Orographic precipitation lowers the moisture content of mP air during its westward flow. Leeward of the Rockies, the air is dry. Chinook winds can form if conditions are favorable.

13 East Coast mP Air A strong anticyclone in eastern Canada creates northeasterly winds that may bring cold, unstable Atlantic mP air and storms into New England and the middle Atlantic States. These storms are known as nor’easters. Figure 12.8

14 Tropical Pacific mT Air
“Pineapple Express” Figure 12.9 Warm and moist maritime air from the tropical Pacific may reach the West Coast as a series of unstable waves, bringing powerful thunderstorms and heavy rains.

15 Gulf & Caribbean mT Air Gulf of Mexico and Caribbean Sea warmth and moisture flows into the East Coast due to a strong anticyclone. When it rises above dense cP air, heavy and widespread precipitation can result. Figure 12.10

16 cT Air Dry, hot air from the Mexican desert can cause low level instability in the U.S. interior during summer, and may trigger dust devils. An upper level ridge of high pressure may add compressionally heated air to the region, enhancing the dry, hot conditions. Figure 12.11

17 Air Masses and Fronts Figure 12.12 -Most “weather” occurs in a narrow “battle zone” located between large air masses…terminology developed during WWI -This transition zone is called a front…boundary of very strong differences in temperature and humidity -Four different fronts are used on weather maps:

18 Types of Fronts Cold front: cold, dry air replacing warmer moist air
Warm front: warm, moist air replacing colder air Stationary front: boundary between air masses is not moving Occluded front: frontal boundary lifted above another frontal boundary

19 Front Identification Locating a front on a weather map involves finding sharp changes in: a) temperature b) dew point c) wind direction d) pressure and e) cloud/precipitation patterns. In this figure, pressure tendency is shown as the line to the right of the station plot Figure 12.13

20 Cold Front

21 Cold Front Animated

22 Two types of Cold Fronts
Anafront: A cold front breeds more precipitation when weak upper-level winds relative to stronger surface winds stretch the cold front out and the slope of the cold air becomes much less. This usually occurs when the upper-level winds become parallel to the front. Katafront: Some cold fronts produce very little or no precipitation as they move across the country. The only sign a front has moved through your area is a sudden change in winds and temperature.

23 Cold Front Transition Figure 12.14 Important cloud, wind, and temperature changes are revealed in this cross-section view of a typical cold front. The front slopes steeply (1km in 50km), and cirriform clouds protrude ahead.

24 Cold Front Clouds

25 Strengthening Front Figure 12.15A Figure 12.15B Satellite imagery shows the transition between a weak front and its frontogenesis, or strengthening, as it moves offshore over warmer water.

26 Back Door Cold Front Eastern Canadian high pressure can generate cold fronts from the northeast, which mix with the warm, moist Gulf air. Cold air damming describes how the Appalachian Mountains confine the front's westward movement. Figure 12.16

27 Warm Fronts Observed wind, temperature, pressure, humidity, clouds, and rain patterns experienced before, during, and after a front. Note the rotation of winds and change in temperature along this warm front. Figure 12.17

28 Warm Front Transition Figure 12.18A Unique clouds and precipitation patterns are associated with warm fronts --A broader range of precipitation than in a cold front. The cross-sectional view shows the gentle slope of overrunning warm air, a typical temperature inversion, and the shifting winds.

29 Warm Front Animation Less dense, warm air gradually climbs surface of cooler air

30 Warm Front

31 Warm Front Clouds

32 Occluded Fronts When a cold front catches up to and overtakes a warm front, the frontal boundary created between the two air masses is called an occluded front or simply an occlusion Represented as a solid purple line with alternating cold front type triangles and warm front half circles Two types: Warm and cold occlusions Cold occlusions most prevalent in the Pacific coastal states Warm occlusions occur when the milder, lighter air behind a cold front is unable to lift the colder heavier air off the ground and instead rides up along the sloping warm front

33 Occluded Fronts Fast moving cold front may overtake the slower moving warm front, particularly when they are influenced by cyclonic winds. Cold occlusion describes this scenario with very cold air overtaking the warm sector

34 Stationary Front A front with essentially no movement
Drawn as alternating red and blue line. Semicircles face toward colder air on the red line and triangles point toward warmer air on the blue line. Winds tend to blow parallel to a stationary front. If either a cold or warm front stops moving, it becomes a stationary front

35 Stationary Front—Parallel Upper Level Winds

36 Fronts, Jets, and Cyclones
Fronts are associated with mid latitude cyclones--which are caused by jet streams.

37 Mid-Latitude Cyclone Occluded fronts are common along mid-latitude cyclones, or deep low pressures centers about which the cold and warm fronts pivot. These storms appear frequently in the mid-latitudes. Figure 12.21

38 Upper Air Front A 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. Figure 12.22

39 Midlatitude Life Cycle (Continued in
next chapter)

40 Key Facts and Concepts An air mass is a large body of air with similar horizontal temperature and moisture characteristics Regions where air masses originate and acquire their properties of temp and moisture are called source regions. These regions are generally flat, of uniform composition, where winds are light cP are cold and dry; cA extremely cold and dry; cT hot and dry; mT warm and moist; and mP cool/cold and moist A front is a sharp transition zone that separates two air masses with contrasting meteorological properties A cold front is a region where cold air is replacing warmer air. Along the leading edge of the cold front, warm air is forced rapidly upward, producing a narrow band of showery precipitation

41 Con’t The cold front represents a trof of lower pressure where there is a wind shift (most often from SW to NW). After the passage of a cold front, temperatures drop and pressures rise A warm front is a region where warmer air is replacing colder air. At the warm front, the warm air slowly rises up and over the colder air—producing stratiform clouds and continuous precip ahead of the warm front As the warm front approaches, the clouds often change from high clouds (Ci and Cs), to middle clouds (As) to low clouds (Ns and St) An occluded front often has the characteristics of both a cold front and a warm front


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