An air mass is a vast body of air that possesses generally uniform temperature and moisture characteristics throughout its horizontal extent. These massive air parcels, which can stretch across hundreds or thousands of miles, are the fundamental building blocks of weather systems. They acquire their distinct properties from the surface of their source region, a process that takes days or weeks of stagnation over a uniform area. Their movement drives the global transport of heat and moisture, determining the day-to-day weather we experience.
Understanding the Classification Code
Meteorologists use a specific two-letter code to classify air masses based on their source region and properties. The first, lowercase letter indicates the air mass’s moisture characteristic, determined by whether it forms over land or water. The letter ‘c’ stands for continental, meaning the air mass forms over land and is dry. Conversely, ‘m’ denotes maritime, signifying the air mass originated over an ocean and is moist.
The second, uppercase letter describes the air mass’s temperature, which relates directly to the latitude of its source region. ‘T’ represents Tropical, indicating a warm air mass that forms in low latitudes near the equator. ‘P’ stands for Polar, identifying a cold air mass that develops in high-latitude regions. A third designation, ‘A’ for Arctic or Antarctic, is sometimes used for air masses that are extremely cold.
The Four Primary Air Mass Types
The combination of the moisture and temperature designations creates the four air mass types that frequently affect North America and other mid-latitude regions. These types are Continental Polar (cP), Maritime Polar (mP), Continental Tropical (cT), and Maritime Tropical (mT). Each combination brings a predictable set of conditions that define the regional weather.
Continental Polar (cP)
Continental Polar air masses originate over the high-latitude landmasses of interior Canada and Siberia, especially during the winter. Contact with the snow-covered, frozen ground in these source regions makes the air cold and dry. This air is stable, meaning it resists vertical movement, which results in clear, cloudless skies and very low temperatures. When a cP air mass moves southward, it is responsible for the clear, biting cold of winter cold waves.
Maritime Polar (mP)
Maritime Polar air masses develop over the northern Pacific and Atlantic Oceans in the high latitudes. While cool due to their origin, they gain moisture as they pass over the relatively warmer ocean waters. This process often causes the lower layers of the air mass to become unstable, leading to cloudy, damp conditions. An mP air mass commonly brings cool temperatures, fog, and light drizzle or low clouds to coastal regions.
Continental Tropical (cT)
Continental Tropical air masses form over hot, low-latitude desert regions, such as the arid lands of the southwestern United States and northern Mexico. Prolonged exposure to the hot, dry land surface makes this air mass hot and dry. While the air near the surface is often unstable due to intense solar heating, the overall air mass is characterized by high temperatures and low humidity. The arrival of a cT air mass is associated with periods of drought and heat waves.
Maritime Tropical (mT)
Maritime Tropical air masses originate over warm subtropical ocean waters, including the Gulf of Mexico, the Caribbean Sea, and the tropical Pacific. These air masses are warm and carry water vapor, making them exceptionally moist. The warmth and high moisture content make the air mass unstable, leading to rising air motions, cloud formation, and precipitation. The influx of mT air masses is the cause of high summer humidity, heavy rainfall, and thunderstorm activity across the eastern two-thirds of North America.
Air Mass Modification and Resulting Weather
An air mass begins to change its properties as soon as it moves over a surface with different characteristics. This process, known as air mass modification, occurs when the air mass exchanges heat and moisture with the new underlying surface. For instance, a cold, dry cP air mass moving over the comparatively warmer waters of the Great Lakes in winter will absorb heat and moisture from the water. This modification can destabilize the air, leading to localized weather phenomena like lake-effect snow downwind of the water body.
A warm, moist mT air mass moving northward over a colder land surface will be cooled from below, which tends to make the air more stable. The interaction between different air masses is the primary driver of day-to-day weather changes. When two air masses with contrasting temperature and moisture characteristics meet, they do not easily mix; instead, they form a distinct boundary known as a front. This convergence creates zones of rising air and atmospheric lift, which are the main mechanisms responsible for cloud development, precipitation, and significant changes in wind and temperature.