When the boundary zone between two contrasting air masses is sharply defined, it is called a frontal zone, and the edge of the frontal zone that is next to the warmer air is called the front. Fronts are identified by their direction of motion. A cold front is formed when a mass of cool air displaces a mass of warmer air. The cool air wedges its way beneath the warm air as it advances because cool air is heavier than warm air. A warm front is formed when a mass of warm air displaces a mass of cooler air. The warm air moves up over the retreating wedge of the cool air mass, often producing a line of storms.

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This page was last updated on 12/15/2017.