Chapter 8 Notes

Air Masses

Maritime Tropical Air Mass Brings Summer Storms over Sonora Desert

Weather patterns are the result of the movements of large bodies of air called air masses

An air mass is an immense body of air, usually 1600 kilometers (1000 miles) or more across and perhaps several kilometers thick. They are characterized by homogeneous physical properties at any given altitude. When the air moves out of its region of origin, it will carry these temperatures and moisture conditions elsewhere, effecting a large area as it moves.

Air-Mass Source Regions for North America

Source Regions

Areas in which air masses originate are called source regions. The nature of the source region largely determines the initial characteristics of an air mass. An ideal source region must meet two essential criteria:1)It must be an extensive and physically uniform area2)The area must be characterized by a general stagnation of atmospheric circulation so that air will stay over the region long enough to come to some measure of equilibrium with the surface

The source regions that produce air masses that most often influence North America in the region that includes:1) the Gulf of Mexico and the Caribbean Sea, and the Pacific west of Mexico that produce warm, humid air masses.2)The North Pacific and north Atlantic that produce cold, humid air masses3)The Northern part of the North American continent the produces cold, dry air masses

Source regions are not found in the middle latitudes but instead are confined to subtropical and subpolar regions

Classifying Air Masses

The classification of an air mass depends on the latitude of the source region and the nature of the surface in the area or origin—ocean or continent.

Air masses are identified by two-letter codes. With reference to latitude (temperature), air masses are placed into one of three categories: polar (P), arctic (A), or tropical (T).

To designate the nature of the surface of the source area, the lowercase m (maritime) and c (continental) are used.

When this scheme is applied, the following air masses can be identified:

cA continental arctic

cP continental polar

cT continental tropical

mT maritime tropical

mP maritime polar

Air Mass Modification

After an air mass forms, it normally migrates to a region with different surface characteristics than the source region. Two thing then happen:1)The air mass modifies the weather of the area it is traversing, and2)The air mass is modified by the surface over which it is moving

The air mass is changed by warming or cooling from below, the addition or loss of moisture, and the vertical movements of air within the mass

When an air mass is colder than the surface over which is it passing, the lowercase k is added after the air-mass symbol. If the air mass is warmer than the underlying surface, the lowercase w is added.

The k or w designation gives an indication of the stability of an air mass and also the weather that might be expected.

Air masses frequently pass over us, which means that the day-to-day weather we experience often depends on the temperature, stability, and moisture content of these large bodies of air. The table above summarizes the properties of the principal North American air masses.