Fronts are weather systems that are sometimes called waves. Along the front, two air masses of widely different characteristics fight a battle for supremacy. Usually the colder of the two masses, being heavier, predominates, forcing the warm air upward. Cold air behind a cold front displaces the warm air ahead of it upward. The warm air behind a warm front moves upward over a retreating cold air mass. When a cold front moves faster than the warm front, it overtakes the warm front, forcing the warmest air masses upward. When these fronts converge, the remaining front on the surface is called an occluded front.
A cold front or a warm front may extend for hundreds of miles long, but the area in which frontal weather disturbances take place is usually a band 15 to 50 miles wide for a cold front and up to 300 miles for a warm front. The point where the cold and the warm fronts converge is frequently the center of a low-pressure area.
When a cold front is coming your Way, the first change you notice is darkening of the horizon to the west and to the north. Very soon the cloud ceiling lowers and rain begins. A fast-moving cold front (which can move 720 miles in a day), with typical cumulonimbus clouds preceding it, brings sudden violent showers or thunderstorms. If the cold front is not preceded by cumulonimbus clouds, the rainfall is steady. Passage of the cold front is usually marked by a wind shift, a drop in temperature, a rise in pressure, and a rapid clearing of the sky condition and visibility.
A warm front, headed by cirrus clouds, is followed (in order) by cirrostratus, altostratus, then nimbostratus and possibly stratus clouds. Visibility is poor in advance of a warm front; frequently fog forms and steady rain or drizzle prevails. Thunderstorms may develop ahead of this front. The frontal line is passing when a marked shift occurs in the wind direction, and the temperature of the atmosphere rises sharply. Gradual clearing takes place and remains steady or falls slowly.
