Thickening cumulus clouds warn of the coming cold front, where cold polar air cuts in sharply beneath the warm, moist tropical air. The cold front slopes much more steeply than the warm front, and strong updrafts can stir up violent storms. Huge cumulonimbus many build up all along the front, bring heavy rain and sometimes thunderstorms as it passes over.

 Warm, moist air from the tropics slides over a wedge of cold polar air. As the warm air moves over the cold air. Wispy cirrus clouds form and a milky veil of cirrostratus clouds can be seen. Above the base of the front, clouds thicken first with altostratus above great, gray nimbostratus clouds. A rain storm falls in the cold sector beneath the front.