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Ch 11

Terms in this set (21)

Air Mass
an extremely large body of air whose properties (temperature and humidity) are uniform
source region
The region where a given air mass originates (ex. icy arctic plains and subtropical oceans)
The mid-latitudes are not good source regions because temperature and moisture vary greatly
source region characteristics
Flat
Uniform composition
Light surface winds
The longer air stays there, the more properties of the region it will acquire
Usually dominated by high surface pressure
Air Mass Classification
Air masses are named based on their temperature and humidity characteristics (source region)
Temperature Organization
Arctic (coldest)
Polar (cold)
Tropical (warm)
Humidity/Source
Continental
(dry/land)
Maritime
(moist/water)
Continental Arctic/Polar air mass
In the winter, this air mass brings dry, frigid air south into the U.S.
This can produce lake-effect snow, when cold air blows over relatively warm lakes in early winter
In the summer, the source region experiences long days which melt snow and warm the land
Air Mass Modification
Lake-effect snow is an example of modification of continental polar air masses
Air blowing over a lake warms significantly when the water is much warmer
The rest of the air mass being cold, the air becomes buoyant and rises
With the addition of water vapor from the lake, snow storms form leeward of the lake
Maritime Polar
Because mP air masses come from water (they are moist) and polar regions (they are cold), they have high relative humidity

Rain/snowfall can be significant if this air is lifted
Various mountain ranges in the west or the Appalachians in the east will do this
Maritime Tropical
Air originating from tropical oceans is warm and humid
Lots of water vapor content

This air easily produces cumuliform clouds in areas like the Gulf, which contribute to daily afternoon showers and thunderstorms
Continental Tropical
Dry, hot air originating from northern Mexico
Low relative humidity means that condensation can't occur until after several kilometers of lift
High pressure aloft (remember: warm air column) produces sinking motion, so condensation is rarer yet
Fronts
Front - the transition zone between two air masses of different densities.
Temperature differences cause density differences.
Fronts often separate air masses with different temperature characteristics.
Fronts can also separate air masses with different humidity characteristics.Ex.: Dry line
Fronts are named for the air mass replacing the other
Cold air replacing warm air = cold front
Fronts are usually analyzed at the surface, but they can have significant vertical extent
Lines are drawn on the map at the leading edge of the transition zone
Frontal Strength
A front is usually defined by sharp temperature gradients between air masses
We consider its "strength" to be related to this
Frontogenesis
the formation or strengthening of a front
Increasing the temperature gradient
Frontolysis
the weakening or dissipation of a front
Decreasing the temperature gradient
Common Types of Fronts
Stationary front
Cold front
Warm front
Occluded front
Dry line
Stationary Front
Moves little or not at all (stationary)
Separates different types of air masses
Winds generally blow parallel to stationary front, but opposite directions on either side of it
Can be associated with dry weather or widespread precipitation
Depends on the types of air masses it is separating
cold front
A zone where cold air replaces warm air
The warm air is lifted up and over the encroaching cold, stable air
Because of friction, cold fronts are vertically steep with strong upward forcing
Heavy rain and thunderstorms are common along and just ahead of a cold front
warm front
A zone where cold air retreats and warm air advances
Warm air rising up and over the cold air in this way is called overrunning
Steady, light to moderate precipitation is typically found ahead of or along a warm front
occluded front
When a cold front catches up and merges with a warm front, the result is an occluded front
A cold occlusion forms when the warm front rides up and over the cold front
Air behind cold front colder
A warm occlusion forms when the cold front rides up and over the warm front
Air ahead of warm front colder
dry line
A front that separates air of different humidities.
Commonly found in Texas in the spring, separating continental tropical and maritime tropical air
Thunderstorms do not always develop along dry lines, but when they do, they are often severe
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