Air Masses And Fronts

AVIATION WEATHER

CHAPTER THREE

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