There are five air masses that effect weather in the United States during a typical year. “Air masses are relatively large bodies of air that are fairly horizontally uniform in characteristics. They have relatively uniform temperature and moisture content; the region separating two different air masses is called a front.” (Oklahoma Climatic Survey, para 2).
The characteristics of air masses are determined by the land underneath the air mass. “Air masses form in "source regions" where there is little topography and relatively stagnant winds near the surface. The air mass takes on the properties of the surface of the source region (e.g., dry, hot, moist, etc.). It takes several days for an air mass to "form", so they generally form in areas of high pressure (light winds).” (Oklahoma Climatic Survey, para 3).
Continental polar air masses are made up of cold and dry air, but stable. This type of air mass generally forms over northwestern Canada and Alaska. They then move south and east to the northwestern and west central part of the United States. The air mass usually flows through the states of Wyoming, Montana, and eastern Colorado. The air mass then usually moves further east, pushed by the winds. How far south this type of air mass will travel depends a lot on how far the jet stream dips.
Continental Arctic air masses are also made up of cold, dry and stable. This type of air mass also generates over Canada and moves southward to the United States, similarly to the continental polar air mass, the difference is that the continental Arctic air mass is much colder than a continental polar air mass. Continental Arctic air masses produce the below zero weather usually seen in the north central and mid-western parts of the United States in the winter months.
Maritime polar air masses are made up of cold and wet air and unstable. This type of air mass can effect two different parts of the United States, depending on where they form. This type of air mass often starts over Asia and move towards the western coast of the United States over the Pacific Ocean. This air mass picks up some warmth over the ocean. It comes onto the northwestern coast of the United States and usually dumps a lot of moisture over the northern California, Oregon and Washington coasts. Maritime polar air masses can also develop over the Northern Atlantic and move westward toward the northeastern United States. This type of maritime polar air mass is usually colder and drier than its Pacific coast counterpart.
Maritime tropical air masses are made up of warm, moist air and are very unstable. This type of air mass comes into the southern part of the California coast after having travels hundreds of miles over the open Pacific Ocean. This type of air mass usually does not extend beyond southern California and northern Mexico because it is stopped by the mountains and deserts surrounding the coast areas. Southern California usually gets a lot of rain when these air masses come on the coast. Maritime tropical air masses also develop in the Gulf of Mexico and the Caribbean Sea during the spring and summer months. These air masses travel north and are usually what starts thunderstorms in the Great Plains and southeastern United States.
Continental tropical air masses are made up of hot and dry air. They are unstable and have been known to cause droughts. They form in northern Mexico and travel north to southwestern states like west Texas, New Mexico and Arizona. Occasionally the air masses will travel further north east into the Great Plains.
These air masses can change dramatically as they move over land and water and run into other air masses coming in from different regions. They are a big part of what make up the weather patterns in the United States and have to be watched closely by meteorologists who are trying to predict what kind of weather the air masses are going to cause.
Haslam, Andrew and Barbara Taylor. Make It Work: Weather. 2001. Two-Can Publishing: London