What is a cold front?

Dateline article| David Murphy|

by David Murphy

A cold front is the leading edge of a mass of cool air, as it moves into a warmer region. A cold front is drawn on a weather map as a blue line with pointed pips or triangular projections pointing in the direction of the front's movement. Cold fronts almost always move from west to east or from north to south. This is due to the direction of the earth's rotation and the resulting general flow of winds.

Cold fronts are the product of low pressure systems which form along flat, stagnant boundaries separating cold and warm air masses. As the low forms, it begins to rotate thanks to forces in the atmosphere like air pressure and gravity. As it rotates, the low draws cold air around one side and warm air around the other. Once this air is on the move, you officially have fronts. A cold front is usually pictured on a weather map with its top end extending out from the low that fathered it.

A cold front can move quickly or slowly and can be either strong or weak, depending on how much the air behind the front differs from the air ahead of it. For example, a vastly cooler, drier air mass that heads into a drastically warmer, wetter air mass will produce a strong front. Strong fronts tend to produce more clouds, precipitation and storms. Weak fronts, which mark the boundary between two very similar air masses, may only produce a few clouds and nothing more.

The wind ahead of a front always flows along its face, usually from the west or southwest. The air behind the front always pushes in from behind, usually from the west or north. The exact direction of these varying winds depends on the orientation of the front (whether it's lined up from north to south, or from west to east). The fact that wind direction is fairly predictable on either side of a cold front often surprises people---but as you can imagine, it's a pretty handy thing to know. One easy way meteorologists identify the location of cold fronts is by looking at the dozens of observation stations on a weather map and spotting where the wind is changing direction.

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