A weather front results when two different air masses meet. Weather changes dramatically along a front. Weather typically stabilizes after a front passes.
The classification of fronts is straightforward. If cold or dry air overtakes warm or moist air, then it is called a "cold front." Conversely, if warm or moist air overtakes cold or dry air, then it is called a "warm front." The common element in classification is density of the air mass. If a dense air mass overtakes a less dense air mass, then the front is classified as a cold front. If a less dense air mass overtakes a more dense air mass then the front is a warm front (See illustration below).
A warm front pushes aside a more dense air mass. Warm fronts are by definition less dense and therefore lighter per unit volume and travel slower (20-30 miles per hour). The less dense air cannot effectively push the heavier air out of the way. Therefore warm fronts slowly rise over cold fronts. As the warm air rises it cools, and then a storm begins, producing light to moderate rain or snow.
A cold front pushes aside a less dense air mass, typically moving at 40-50 miles per hour. The less dense air is rapidly forced up by the cold dense air (see illustration below). A storm quickly develops and brings heavy rain, sometimes hail, and thunder and lightening. A storm from a cold front is usually short lived.