Air Masses and Fronts

The movement of air masses across the Earth's surface is an important component of the weather that develops in an area. For example, weather patterns in North America are largely dominated by the movement of about a half dozen air masses that travel across the continent on a regular basis.

Two of these air masses are the cP and cA systems that originate in Alaska and central Canada and sweep down over the northern United States during the winter months. These air masses bring with them very cold temperatures, strong winds, and heavy precipitation, such as the snowstorms commonly experienced in the Great Lakes states and New England. The name "Siberian Express" is sometimes used to describe some of the most severe storms originating from these cP and cA air masses.

From the south, mT air masses based in the Gulf of Mexico, the Caribbean, and western Atlantic Ocean move northward across the southern states, bringing hot, humid weather that is often accompanied by thunderstorms in the summer.

Weather along the western coast of North America is strongly influenced by mP air masses that flow across the region from the north Pacific Ocean. These masses actually originate as cP air over Siberia, but are modified to mP masses as they move over the broad expanse of the Pacific, where they can pick up moisture. When an mP mass strikes the west coast of North America, it releases its moisture in the form of showers and, in northern regions, snow.