To understand fronts, we must first define and understand what an air mass is. An air
horizontally and vertically uniform. When air stagnates over certain regions, it acquires
properties from the underlying surface (source region) and forms an air mass. The
prevailing weather over any area at any given time generally depends on the properties
and characteristics of the prevailing air mass. In time, these air masses move out of
their source regions because of the general circulation of the earth's atmosphere, the
terrain, and other factors. In the northern hemisphere, cold air masses from the Polar
Regions tend to move southward while warm air masses from the tropical regions tend
to move northward.
When two different air masses meet, the boundary or surface that separates these air
masses is called a front.
Fronts are generally classified according to the relative motions of the air masses
involved. The four chief classifications and their descriptions are contained in Table 1-7.
The weather associated with fronts and frontal movement is called frontal weather. It is
more complex and variable than air mass weather. The type and intensity of frontal
weather is determined by a number of things (i.e., slope of the front, water vapor
content, and stability of the air mass) and may range from a minor wind shift with no
clouds or other visible weather activity to severe thunderstorms accompanied by low
clouds, poor visibility, hail, and severe turbulence and icing. Consider each of the frontal
categories and the weather pattern each usually produces.
A front whose motion is such that cold air displaces warm air at the
A front whose motion is such that warm air replaces cold air at the
A front that has little or no motion
A complex front resulting when a surface cold front overtakes a warm
Table 1-7 -- Four frontal classifications