An air mass is an immense body of air that stretches across thousands of square miles and maintains relatively uniform temperature and moisture characteristics. These large atmospheric bodies acquire their specific properties from the surface of the region where they form, known as the source region. The longer an air mass remains stationary over its source region, the more completely it adopts the traits of that underlying surface. Meteorologists categorize these systems based on their origin, which determines the weather conditions they will carry.
Understanding Air Mass Classification
Meteorologists use a straightforward, two-part letter coding system to classify air masses, which helps to predict the type of weather a region will experience. The first letter indicates the moisture content, determined by whether it forms over land or water. A lowercase ‘c’ stands for continental (dry), while a lowercase ‘m’ stands for maritime (moist).
The second letter denotes the air mass’s temperature, determined by the latitude of the source region. An uppercase ‘P’ signifies Polar, meaning the air is cold or cool because it originated at high latitudes. Conversely, an uppercase ‘T’ stands for Tropical, meaning the air is warm or hot because it formed at low latitudes near the equator. Combining these two elements provides the four distinct air mass categories.
The Four Primary Air Mass Types
The combination of moisture and temperature properties yields four primary air mass types. Each type brings a distinct set of weather conditions to the regions it affects.
Continental Polar (cP)
A Continental Polar (cP) air mass is cold and dry, forming over high-latitude land areas like the interior of Canada or Siberia. This air is stable and brings frigid temperatures and clear skies when it moves south during the winter season. The low moisture content results in minimal precipitation, often leading to cold snaps across the affected areas.
Maritime Polar (mP)
Maritime Polar (mP) air masses are cool and moist, originating over the cold waters of high-latitude oceans, such as the North Pacific or North Atlantic. As this air moves, it carries high humidity and can be unstable, especially when moving over warmer land. Regions affected by an mP air mass often experience cool temperatures, overcast skies, fog, and light, steady precipitation.
Continental Tropical (cT)
The Continental Tropical (cT) air mass is hot and dry, developing over low-latitude land areas like the deserts of the Southwestern United States or the Mexican Plateau. This air is warm due to the intense solar heating of the land surface and contains very little moisture. When a cT air mass dominates a region, it brings clear skies, extreme heat, and drought-like conditions, especially during the summer.
Maritime Tropical (mT)
Maritime Tropical (mT) air masses are warm and moist, forming over low-latitude ocean surfaces, such as the Gulf of Mexico or the Caribbean Sea. This air mass is often unstable, carrying high levels of water vapor due to evaporation from the warm ocean. The mT air mass is a source of warm temperatures, high humidity, and heavy precipitation, frequently leading to thunderstorms and severe weather.
How Air Masses Change Over Time
Air masses are not static; they continuously undergo modification as they travel away from their source regions and move across different surfaces. This process occurs as the air gains or loses heat and moisture from the underlying land or water. For example, a cold, dry cP air mass moving over the warmer water of the Great Lakes in winter will pick up heat and moisture, gradually altering its characteristics.
The movement of air masses is primarily driven by upper-atmosphere winds, such as the jet stream, and large-scale pressure systems. When two air masses with different densities converge, they do not mix easily. Instead, they form a boundary called a weather front. These frontal boundaries are the principal cause of most weather phenomena, as the collision of warm and cold air forces the warmer, less dense air to rise, leading to cloud formation and precipitation.