adverse weather occurs along fronts. In some cases, very large areas of low ceilings and poor
visibility occur in areas far removed from a front.
Air Masses And Fronts
Having introduced the basics of both air masses and fronts, an analysis of a real-world situation
can help explain how these pieces fit together. Figure 3-5 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 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 Midwest/East and Deep South 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 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, there is a cold front.
Mechanics of Frontal Systems 3-5