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|  CHAPTER THREE
AVIATION WEATHER
2.39
Describe the conditions associated with occluded fronts.
2.40
Describe the conditions associated with an inactive front.
302.
REFERENCE
Weather for Aircrews, AFH 11-203, Volume 1, Chapter 8
303.
STUDY ASSIGNMENT
Review Chapter Three and answer the Study Questions.
304.
AIR MASSES
No discussion of fronts would be complete without considering the air masses in vicinity of the
fronts. The various characteristics of a front are highly dependent on the contrasts of the air
masses separated by the front. If these characteristics are not large differences between the air
masses, the front will be weak or non-existent.
An air mass is a large body of air that has essentially uniform temperature and moisture
conditions in a horizontal plane, meaning there are no abrupt temperature or dew point changes
within the air mass at a given altitude. It may vary in size from several hundred to more than
several thousand square miles.
Air masses are named according to their moisture content, location, and temperature (Figure 3-1).
The location of an air mass has a large influence over the other two properties.
Symbol
Source Region
60 - 90 N
A
Arctic
40 - 60 N
P
Polar
South of 30 N
T
Tropical
+ 10 of equator
E
Equatorial
Figure 3-1 Northern Hemisphere Air Mass Source Regions
NOTES
1. Source region never changes. Example: an arctic air mass that
moves South of 30 N latitude is still considered arctic.
2. Arctic air masses form around the North Pole. Polar air masses
do not form at the North Pole.
3-2
Mechanics of Frontal Systems
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