JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
narrow, and in some cases, the zone can be less than a mile wide. On the other hand, gradual
changes indicate the frontal zone is broad and diffuse, often over 200 miles in width. Abrupt
changes will bring more severe weather than gradual changes.
Aviation weather hazards are not limited to the area of frontal zones. Some fronts do not produce
clouds or precipitation. Additionally, weather associated with one section of a front is frequently
different from the weather in other sections of the same front. Do not conclude that all adverse
weather occurs along fronts. In some cases, very large areas of low ceilings and poor visibility
occur in areas that are far removed from a front.
Air Masses and Fronts
Now that we have introduced the basics of both air masses and fronts, an analysis of a real-world
situation can help show how these pieces fit together. Figure 3-2 shows the weather across the
U.S. at the same time from three different points of view. From the frontal systems shown on the
Current Surface chart, we can see that there are three major air masses over the nation: one over
the West, one over the Midwest and the East, and one over the Deep South. For simplicity, we
will compare only the two eastern air masses.
Looking at the Current Temperatures chart, the Midwest air mass (centered approximately on the
“H” of the high pressure) has temperatures in the 50s, give or take a few degrees. So far, this
shows a relatively uniform temperature across the air mass, matching with what we would expect
from the discussion above. The southern air mass, on the other hand, has much warmer
temperatures, generally in the 70s and 80s. Even so, these temperatures are still relatively
uniform throughout the air mass.
The dew points are also different between the two air masses. Even though the Dew Point chart
only indicates dew points above 50° F, it is clear that the southern air mass contains much more
moisture than the air mass to its north. Thus, these charts indeed show two air masses over the
eastern U.S., each with temperature and moisture properties different from the other.
Accordingly, a front has been drawn between the two. From the “L” to just south of the “H,”
there is a warm front, and to the east of that position, all the way to the next “L” over New
England, it is a cold front.
Version 3.2/Dec 08 3-3