An air mass is a large body of air with generally uniform temperature and humidity. The area over which an air mass originates is what provides its characteristics. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.

An air mass forms whenever the atmosphere remains in contact with a large, relatively uniform land or sea surface for a time sufficiently long to acquire the temperature and moisture properties of that surface. The Earth’s major air masses originate in polar or subtropical latitudes.

A system used to identify and to characterize the different air masses according to a basic scheme. A number of systems have been proposed, but the Bergeron classification has been the most widely accepted.

The maritime Tropical (mT) is the most important moisture-bearing and rain-producing air mass throughout the year. In winter it moves poleward and is cooled by the ground surface. Consequently, it is characterized by fog or low stratus or stratocumulus clouds, with drizzle and poor visibility.

Maritime tropical air masses originate over the warm waters of the tropics and Gulf of Mexico, where heat and moisture are transferred to the overlying air from the waters below. The northward movement of tropical air masses transports warm moist air into the United States, increasing the potential for precipitation.

Gulf of Mexico

The warm waters conduct heat toward the low levels of the atmosphere. Temperatures in this air mass warm to highs in the 80’s and 90’s in the summer and the 60’s and 70’s in winter. High dewpoints (generally greater than 50 F) characterize mT air.

An arctic maritime air mass has similar characteristics to a polar maritime air mass, but because of the shorter sea track the air is colder and less moist.

This process is called air mass modification. For example, an mP airmass that moves from the Pacific ocean over the mountains in the western continental US will typically dry as it crosses the mountains, rains out its moisture, and warms over the land surface until it becomes a cT airmass.

High pressure areas are usually caused by air masses being cooled, either from below (for instance, the subtropical high pressure zones that form over relatively cool ocean waters to the west of Califormia, Africa, and South America), or from above as infrared cooling of winter air masses over land exceeds the warming …

Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it. Two air masses meet at a front.

Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator. This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward.

Generally high pressure means fair weather, and low pressure means rain. This weather map shows areas of high and low pressure.