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04/19/2006 - Updated 01:25 PM ET

Source: USA TODAY research, Graphic by John Herne

Cold fronts not always all that cold

By Chad Palmer, USATODAY.com

The term "cold front" is one of meteorology's most misused terms. Many people say "cold front" when they are really talking about the mass of cold air that moves in behind the front. (Related graphic: All about air masses).

In weather, all fronts are boundaries between masses of air with different densities, usually caused by temperature differences.


While a winter cold front can bring frigid air, summer cold fronts often can more accurately be called "dry" fronts. As anyone who's ever suffered through a few days of hot, humid air anywhere east of the Rockies can tell you, cold fronts are welcome visitors because they often bring air that might be only a few degrees cooler, but much less humid.

The weather map symbol for a cold front is a blue line with triangles pointing the direction the cold air is moving.

As a cold front moves into an area, the heavier, cool air pushes under the lighter, warm air it's replacing. The warm air cools as it rises. If the rising air is humid enough, water vapor in it will condense into clouds and maybe precipitation.

In the summer, an arriving cold front can trigger thunderstorms, sometimes severe thunderstorms with large hail, dangerous winds and even tornadoes.

As a cold front arrives in a particular place, the barometric pressure will fall and then rise. Winds ahead of a cold front tend to be from a southerly direction while those behind the front - in the cooler air - tend to be northerly. In fact, weather stations use the shift from a southerly to a northerly wind direction as the indication that a cold front has passed the station.

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