A cold front forms when a mass of cold air pushes under warmer air ahead of it.

The longer version

A front is the boundary between two air masses of different temperature and density. Cold air is heavier, so as it advances it slides beneath the warmer air and forces that lighter air to rise along a steeply sloping surface.

The forced lifting cools the warm air quickly, and its moisture condenses into a narrow line of cloud and showers. Because the slope is steep, the weather arrives suddenly, and the passage is marked by a sharp temperature drop, a shift in wind direction, and clearing skies behind it. Showers along the front are often heavy but brief, since the narrow band of lifting passes through quickly.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.