Description

When large masses of warm air and cold air meet, they do not mix because of density differences. In World War I, meteorologists in the “Norwegian School of Meteorology” referred to this boundary as a “front”, a military analogy to a battle line where in this case the warm and cold air masses were “fighting” for supremacy.

Fronts are usually hundreds of miles long. A cold front forms when a cold, dense air mass pushes under a warm, lighter air mass, forcing the warm air to rise. The cold air advances, replacing the warm air at the surface. Rain and even thunderstorms can form as the moisture in the warm air mass rises, cools, and condenses. With a cold frontal passage, the winds will typically shift from the south to the west or north. As the front moves through, cool, fair weather is likely to follow.