What Are the Causes of a Cold Front?

Cold Front Air Masses

• The first ingredient for a cold front is a cold air mass. Air masses are large bodies of air with uniform temperatures, in this case cold, and humidity. There are three types of air masses associated with cold fronts. Maritime polar air masses are cold and moist. They develop over the North Pacific and North Atlantic. Continental polar air masses are cold and dry and develop over Canada. Continental arctic air masses are also cold and dry, but they develop over the polar cap. These air masses can move over the North Pacific or North Atlantic and absorb moisture. When they do, they become maritime polar air masses.

Low Pressure Systems

• Cold air masses are typically associated with the formation of a low-pressure system, represented by a red "L" on a weather map. With a low-pressure system, air is drawn in at low levels and rises at the center. Once it rises, it forms an outflow in the upper levels of the atmosphere. As the air rises, it causes the entire low pressure system to circulate counterclockwise. On the backside of the low, to the west, this circulation pushes the cold air mass to the south-southeast, around the low. On the eastern side of the low, this circulation draws warm air northward.

Cold Front Formation

• Powered by the low-pressure circulation, the cold air mass collides with the warmer air and begins to replace it. A cold front represents the leading edge of the advancing cold air mass. This front is not a vertically straight wall of cold air. Rather, it is sloped like a ramp. The gradient of this slope is much steeper for a cold front than for a warm front. As the denser cold air plows under the warmer air, this ramp-like front functions like a wedge, rapidly forcing the warm air to rise along the front. This uplift triggers the formation of clouds with vertical development, which usually spawn thunderstorms.

Effects

• Cold fronts can extend outward from the center of low pressure for hundreds of miles. Along this front, you can encounter a variety of severe weather, including gusty wind, heavy rain, damaging hail, lightning, even tornadoes. But the good news is that cold fronts move much more quickly than warm fronts, so while their effects can be extreme, they typically pass fairly quickly. Behind the passing front are clearing skies and cooler temperatures.