Weather Patterns

Air masses are large bodies of air that have similar temperatures and water content throughout. Air masses and their movements determine the regional weather.

Air masses are named and characterized by the regions where they come from—arctic, polar or tropical. Continental air masses are typically dry and originate over continents. Maritime air masses are wet and originate over oceans.

In the United States, wind belts called the westerlies move air masses from west to east. When air masses move and interact with each other, the weather changes. Warm air is less dense, and it will rise when it meets cooler air.

A weather front is the boundary where two different air masses meet. A cold front occurs when a cold air mass runs into a warm air mass and creates clouds, rain and storms. A warm front occurs when warm air moves into a cold region, causing drizzly rain or light snow and an increase in the humidity.

A stationary front forms when two different air masses meet and show little movement. This can cause several days of precipitation. An occluded front occurs when a warm air mass is caught between two cold air masses. The warm air that moves upward is cut off from the ground and may form clouds.