GEOG220 Chapter 8: Defining and Classifying Air Masses
18 May 2018
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Defining and Classifying Air masses
8.1 What is an Air Mass?
● An air mass is a large body of air, usually 1600 kilometers or more across, that is
characterized by homogeneous physical properties at any given alt
○ Implies a large body of air
○ When moves out of its region of origin, carries these temps and moisture
conditions elsewhere, affecting a large portion of a continent
● Air masses that form over land areas in the subtropics are hot and dry
○ Sahara desert is an area where these can form
● The air mass can be modified as it moves (ex as it moves south it gets warmer)
but it also modified the weather in the area over which it moves
● Small differences in temp and humidity from one point to another at same level
are to be expected
○ Still these differences within an air mass are small in comparison to rapid
rates of change that happen across air mass boundaries
● May take several days for an air mass to travel an area, which means that the
region is likely to experience generally consistent weather conditions - called air
mass weather
● Study of air mass is related to study of atmospheric disturbances
8.2 Classifying Air Masses
The basic criteria for an air mass source region and identify the source regions that
influence North America
● Source Regions
○ The large areas in which air masses originate
○ Atmosphere mostly heated from below and gains moisture by evaporation
from surface
■ Nature of the source region largely determines the initial
characteristics of an air mass
○ Ideal source region must meet two essential criteria
■ Extensive and physically uniform area
■ General stagnation of atmospheric circulation so hair stays over the
region long enough to come to some measure of equilibrium or
slow moving anticyclones
○ Areas under influence of cyclones not likely to produce air masses
○ Size of the source regions and intensity of temp change seasonally
● Air mass source regions for North America
Document Summary
An air mass is a large body of air, usually 1600 kilometers or more across, that is characterized by homogeneous physical properties at any given alt. When moves out of its region of origin, carries these temps and moisture conditions elsewhere, affecting a large portion of a continent. Air masses that form over land areas in the subtropics are hot and dry. Sahara desert is an area where these can form. The air mass can be modified as it moves (ex as it moves south it gets warmer) but it also modified the weather in the area over which it moves. Small differences in temp and humidity from one point to another at same level are to be expected. Still these differences within an air mass are small in comparison to rapid rates of change that happen across air mass boundaries.
