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UNIT 5

Weather Causes (4)

SECTION 8. FRONTS

1. General

a. When two different air masses meet, they normally do not mix (unless their

temperatures, pressures, and relative humidities are very similar). Instead, a boundary is
created called a frontal zone, or "front." When warm air moves forward, the colder air mass

projects under the warmer air mass in the form of a wedge. If the boundary is not moving,

the condition is called a "'stationary front."

b.Usually, the boundary moves along the earth's surface. one air mass moves back from a

given area, it is replaced by the other air mass moving forward. This action creates a moving
front. If warmer air is replacing the colder air, the front is called warm, or a warm front If

colder air is replacing the warmer air, the front is called cold, or cold front. Since fronts

normally lie between two areas of differing pressures, wind shifts occur in both types, but the

degree of difference is usually greater in a cold front.

2. Warm Fronts

a. When a warm front moves forward, the warm air slides over the wedge of colder air
ahead of it.See the cross section below.