Synoptic-Scale Air Circulation

The migration of an air mass results in transport of its associated temperature, moisture, and chemical contents. Along boundaries between air masses of differing temperature and moisture content, called fronts, the warmer, less 

FIGURE 4-15a A warm front occurs when a warm air mass overtakes an adjoining cold air mass and rises over it. Adiabatic cooling tends to produce condensation, and ultimately precipitation, along the front. In the situation shown, the warm air mass is unconditionally stable. If the warm air mass is conditionally stable, instability is produced when clouds form along the front, and vertical cloud development (often including thunderstorms) occurs. 

FIGURE 4-15b A cold front occurs when a cold air mass overtakes a warm air mass, which it displaces upward. Again, adiabatic cooling of the rising air tends to produce precipitation. Cold fronts are usually faster moving than are warm fronts they are also narrower, with the slope of the interface between air masses typically about 80 1. 

FIGURE 4-15c An occluded front occurs when a warm air mass rises under the combined influence of two colder air masses, causing extensive cloud development and precipitation. 

Cloud patterns associated with fronts frequently can be seen from the perspective of a satellite, but a person on the ground, who cannot observe a front in its entirety, must infer its presence from the characteristic progressions of cloud types and weather that occur. 

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