WX-201 Chapter 12 Air Masses and Fronts. 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. Regional Weather Patterns.
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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.
Air masses of similar temperature and humidity form above flat, uniform regions with light surface winds.
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.
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.
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.
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.
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.
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.
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.
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.
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.
-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:
Locating a front on a weather map involves finding sharp changes in:
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
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.
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.
Satellite imagery shows the transition between a weak front and its frontogenesis, or strengthening, as it moves offshore over warmer water.
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.
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.
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.
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
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.
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.