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Air Mass Basics
Air masses are large bodies of air that are horizontally uniform in temperature and moisture content. These air masses form over large areas of the Earth's surface, called source regions. Source regions have limited topography and minimal winds, limiting turbulence and allowing the air mass to stabilize. As this stabilization occurs, air masses take on the nature, or properties, of their source region. This process can take several days to develop. There are four primary types of air mass characteristics: (1) cold, moist air; (2) cold, dry air; (3) warm, moist air; and (4) hot, dry air.
Source Regions
Source regions can be broken down by location and surface type. In terms of U.S. weather, polar regions are located north of 60 degrees latitude. Air masses that form here are called polar or arctic air masses. Tropical regions are located within 25 degrees of the equator. Air masses that form here are called tropical air masses. Surface types include continental regions and large bodies of water. Air masses that form over land are called continental air masses; air masses that form over water are called maritime air masses. There are five types of air masses that impact U.S. weather, two of which form over water: maritime polar air masses and maritime tropical air masses.
Maritime Polar Air Masses
Maritime polar air masses, or mP, are typically cold, moist and unstable air masses. They can develop over the North Pacific or North Atlantic. North Pacific mPs typically originate as continental polar air masses over Asia. As these air masses travel over the North Pacific, they acquire moisture and become maritime polar air masses. These air masses affect the northwest U.S. North Atlantic mPs move toward the southwest and affect the northern East Coast. North Atlantic mPs tend to be colder and drier than North Pacific mPs.
Maritime Tropical Air Masses
Maritime tropical air masses, or mT, are typically warm, moist and unstable air masses. There are three source regions for mTs. Pacific mTs, which develop over the subtropical Pacific Ocean, rarely affect weather north or east of southern California. An mT can also develop over the Gulf of Mexico. These air masses move northeasterly and are responsible for much of the severe weather over the Plains during the spring. The Caribbean and subtropical Atlantic are the other source region for mTs. These mTs are instrumental in the development and intensification of hurricanes.