Weather Systems and Severe Weather
9.1 Air Masses and Weather Fronts
An air mass is a large body of air covering a relatively wide area and exhibiting “horizontally uniform properties of moisture and temperature. An air mass originates from the source region and determines the moisture and temperature characteristics of an air mass. For an air mass to develop, the surface of the source region must be relatively flat and uniform in composition (i.e., oceans, deserts, glaciers.), but not a combination. If the air mass stays long enough within the source region, it will begin to develop the characteristics of that source region. An air mass is classified by its temperature and moisture and is identified by using a “letter code” system. (Dastrup, 2014)
The first letter is always lower case and determines the moisture content within the air mass:
- m (maritime) and is moist
- c (continental) and is dry
The second letter is always capital and determines latitude.
- E (equatorial) and is hot
- T (tropical) and is warm
- P (polar) and is cold
- A (arctic) and is very cold
A weather front is a transition zone between two air masses of relatively different densities, temperatures, and moisture. When two air masses come into contact with each other, they do not like to mix well because of their different densities (much like water and oil.) Along a weather front, the warmer, less dense air rises over the colder, denser air to form clouds. There are several types of weather fronts: stationary fronts, cold fronts, warm fronts, and occluded fronts. (Dastrup, 2014)
A stationary front occurs when two contrasting air masses of moisture and temperature connect, but neither of them will ground the other. In many ways, the two air masses are stuck or at a stalemate until one of them begins to give ground to the other. Typically, but not always, stationary fronts produce mild but prolonged precipitation.
Cold fronts are zones separating two distinct air masses, of which the colder, denser mass is advancing and replacing the warmer. The colder, denser air pushes under the warm air, forcing the warm, lighter air upward. If the warm air rising is unstable enough, massive thunderstorms are likely to occur. Cumulous and cumulonimbus clouds are common.
Warm fronts mark the boundary between a warm air mass that is replacing a colder air mass. When a warm air mass advances over a cold air mass, the warm air rises over, but at a gentler rate than a cold front. Since the warm air does not rise as fast as a cold front, more stratus clouds form, and precipitation is not as heavy.
Finally, occluded fronts occur when a cold front overtakes a warm front, causing the warmer air to rise above, and meets up with another cold air mass.