Chapter 9 Air Masses and Fronts Chapter 9 Air Masses and Fronts

A cold front occurs when a wedge of cold air advances toward the warm air ahead of it. A warm front represents the boundary of a warm air mass moving toward a cold one. A stationary front differs in that neither air mass has recently undergone substantial movement. Occluded fronts appear at the surface as the boundary between two polar air masses, with a colder polar air mass usually advancing on a slightly warmer air mass.

In a typical mid-latitude cyclone, cold and warm fronts separated by a wedge of warm air meet at the center of low pressure. Cold air dominates the larger segment on the north side of the system.

Cold fronts typically move more rapidly and in a slightly different direction from the warm air ahead of them. This causes convergence ahead of the front and the uplift of the warm air that can lead to cumuliform cloud development and precipitation. In this example, the cold air (in blue) advances from west to east (notice that the wind speed depicted by the thin arrows increases with height). The warm air (in red) is blowing toward the northeast. The cold air wedges beneath the warm air and lifts it upward.

Warm fronts have gentler sloping surfaces and do not have the convex-upward profile of cold fronts. Surface friction decreases with distance from the ground, as indicated by the longer wind vectors away from the surface (a). This causes the surface of the front to become less steep through time (b).

Warm fronts separate advancing masses of warm air from the colder air ahead. As is the case with cold fronts, the differing densities of the two air masses discourage mixing, so the warm air flows upward along the boundary. This process is called overrunning, which leads to extensive cloud cover along the gently sloping surface of cold air.

The most complex type of front is an occluded front or an occlusion, which refers to closure such as the cutting off of a warm air mass from the surface by the meeting of two fronts. When the cold front meets the warm front ahead of it, that segment becomes occluded, as shown above. The warm air does not disappear, but gets lifted upward, away from the surface. The occluded front becomes longer as more of the cold front converges with the warm front.

Eventually, the cold front completely overtakes the warm front, as shown above, and the entire system is occluded. In this occlusion, the air behind the original cold front was colder than that ahead of the warm front. This is an example of a cold-type occlusion.

Occlusions sometimes occur when the circular core of low pressure near the junction of the cold and warm fronts changes shape and stretches backward, away from its original position. In (a), the cold and warm fronts are joined at the dashed line. At some later time (b), the cold and warm fronts have the same orientation with respect to each other as they did in (a), but both have been pulled back beyond the dashed line. The circular isobar pattern of (a) becomes elongated to form a trough over the occluded region.