Air masses are large volumes of air with uniform temperature and moisture, spanning hundreds to thousands of miles across Earth’s surface. Vertically, they extend from the ground to the troposphere, influencing weather patterns over vast regions.
Key Properties of Air Masses
Air masses acquire their distinct temperature and moisture characteristics from the surface over which they form. This process involves heat and moisture exchanges between the air and the underlying land or water. For example, an air mass developing over a cold, snow-covered landmass will become cold, while one forming over warm tropical oceans will absorb warmth and moisture.
The two primary properties used to classify air masses are their temperature and moisture content. Temperature is categorized as either cold (polar) or warm (tropical). Moisture content is designated as dry if the air mass originates over land (continental) or moist if it forms over water (maritime). These classifications describe the general conditions within the air mass.
How Air Masses Form and Where They Originate
Air masses form in source regions, which are extensive, uniform surfaces like vast oceans or continental plains. For uniform properties to develop, air must remain stagnant over the source region for several days to weeks, allowing it to absorb the surface’s temperature and moisture.
Ideal source regions have high atmospheric pressure and gentle, divergent air circulation, which keeps the air still. Examples include polar regions (generating cold air) and subtropical high-pressure belts over oceans (producing warm, moist air). Mid-latitude areas are generally not source regions due to frequent weather disturbances.
Major Categories of Air Masses and Their Typical Weather
Meteorologists classify air masses using a two-letter system. The first letter (‘c’ or ‘m’) indicates moisture (continental/dry or maritime/moist). The second letter (‘P’ or ‘T’) indicates temperature (polar/cold or tropical/warm). This system helps predict the weather an air mass will bring.
Continental Polar (cP)
Continental Polar (cP) air masses originate over large, high-latitude land areas like northern Canada or Siberia. Cold, dry, and stable, they often bring clear skies and very cold winter temperatures. In summer, cP air masses warm southward, leading to pleasant, dry weather, though scattered showers may occur.
Maritime Polar (mP)
Maritime Polar (mP) air masses form over cold, high-latitude oceans like the North Atlantic and North Pacific. Cool and moist, they often bring cloudy conditions, fog, drizzle, and light rain. Unstable, they produce significant precipitation when forced to rise over terrain.
Continental Tropical (cT)
Continental Tropical (cT) air masses develop over low-latitude desert regions like the Sahara Desert and southwestern United States. Extremely hot and dry, they are the most arid. They typically bring clear skies, intense heat, and minimal rainfall, sometimes leading to drought.
Maritime Tropical (mT)
Maritime Tropical (mT) air masses originate over warm tropical oceans like the Gulf of Mexico and the Caribbean Sea. Warm, moist, and often unstable, they carry significant water vapor. They cause high humidity, cloudiness, and substantial rainfall, including widespread thunderstorms, especially during warmer months.