JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Air masses are named according to their moisture content, location, and temperature. The
location of an air mass has a large influence over the other two properties. Naturally, moist air
masses will have a greater potential for producing clouds and precipitation than dry air masses.
Most importantly, though, its temperature indicates the stability of the air mass. Warm air masses
bring stable conditions, while cold air masses are inherently unstable.
Frontal Systems
A front is an area of discontinuity that forms between two contrasting air masses when they are
adjacent to each other. A front can be thought of as a border, boundary, or line between the air
masses. These air masses must have sufficiently different temperature and moisture properties–
the defining characteristics of an air mass–otherwise there would be little reason to distinguish
between them. Since air masses cover many thousands of square miles, the boundary between
them can be hundreds of miles long. As air masses are three-dimensional, fronts are, as well. The
point where a front comes in contact with the ground is called the surface front. The surface front
is the line that is plotted on surface analysis charts with different colors and shapes representing
each type of front, as pictured in Table 3-1.
Type of Front Color Scheme Symbol
Cold front
Warm front
Occluded front
Stationary front
Trough
Ridge
Squall line
Blue
Red
Purple
Blue and Red
Brown or Black
Yellow or Black
Purple
Table 3-1 — Frontal Symbols
The frontal zone is that area that encompasses the weather located on either side of the front. The
depth of this frontal zone depends on the properties of the two air masses. When the properties
differ greatly, the resulting narrow frontal zone can include sudden and severe weather changes.
It is often impossible to determine the exact outer boundaries of a frontal zone.
Most active weather is focused along and on either side of the surface front and frontal zone.
Likewise, most aviation weather hazards are also found in the vicinity of fronts. In the midlatitudes,
fronts usually form between the warmer, tropical air to the south and the cooler, polar
air to the north.
When a pilot passes through a front, or a front moves past a station, the atmospheric conditions
change from one air mass to those of the other. Abrupt changes indicate that the frontal zone is
3-2 Version 3.2/Dec 08