OVERVIEW Describe slow-moving cold fronts, fast-moving cold fronts, secondary cold fronts, and cold fronts aloft. Describe instability and squall lines and their relationship to cold fronts.

A cold front is the leading edge of a wedge of cold air that is underrunning warm air. Cold fronts usually move faster and have a steeper slope than other types of fronts. Cold fronts that move very rapidly have very steep slopes in the lower levels and narrow bands of clouds that are predominant along or just ahead of the front. Slower moving cold fronts have less steep slopes, and their cloud systems may extend far to the rear of the surface position of the fronts. Both fast-moving and slow-moving cold fronts may be associated with either stability or insta-bility and either moist or dry air masses.

Certain weather characteristics and condi-tions are typical of cold fronts. In general, the temperature and humidity decrease, the pressure rises, and in the Northern Hemi-sphere the wind shifts (usually from south-west to northwest) with the passage of a cold front. The distribution and type of cloudiness and the intensity and distribu-tion of precipitation depend primarily on the vertical motion within the warm air mass. This vertical motion is in part dependent upon the speed of that cold front.

Learning Objective: Describe slow-moving cold fronts, fast-moving cold fronts, secondary cold fronts, and cold fronts aloft .

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