An air mass is a large volume of air, often spanning hundreds or thousands of miles, that possesses relatively uniform temperature and moisture characteristics. Air masses acquire their specific properties by remaining stationary over a particular region for several days or weeks. Meteorologists classify them based on the source region’s latitude, which determines temperature, and the underlying surface, which dictates moisture content. Continental Tropical (cT) is a specific type of air mass that plays a significant role in summer weather across North America.
Defining Continental Tropical Air
The Continental Tropical (cT) air mass is defined by two primary characteristics: extremely high temperatures and very low moisture content. The “continental” designation signifies its dry nature, as it forms over land and cannot acquire significant water vapor. The “Tropical” designation indicates its origin in low-latitude regions, resulting in intensely warm air.
This air mass is associated with high surface temperatures that regularly exceed 95°F and often climb past 100°F during the day. The relative humidity within cT air is exceptionally low, frequently registering less than 30%. Intense solar heating makes the air near the surface very unstable and buoyant. This combination of heat and dryness contributes to large daily temperature ranges, with daytime highs dropping significantly at night.
Primary Source Regions and Formation
The primary source region for Continental Tropical air in North America is centered over the arid and semi-arid lands of the Southwestern United States and Northern Mexico. This air mass forms over desert regions, including the Mexican Plateau, the Chihuahuan Desert, and the Desert Southwest of the U.S. These regions provide the expansive, flat, and dry land surface necessary for the air mass to acquire its continental properties.
The formation process requires intense solar radiation over the low-latitude land surface, which warms the air from the ground up and gives it tropical temperature characteristics. Crucially, the region must be dominated by a persistent high-pressure system, common in the subtropics during summer. This high-pressure area causes air to slowly sink (subsidence), which warms the air further and suppresses cloud formation. The subsiding air acts like a lid, keeping the air dry and allowing the surface to heat unimpeded. This process takes the weeks required for the air mass to fully acquire its characteristic cT properties.
Seasonal Influence and Movement
The Continental Tropical air mass is predominantly a summer phenomenon, typically influencing weather patterns from late spring through early fall. It rarely forms in winter because the sun angle is too low to provide the necessary intense surface heating. Once established, the cT air mass tends to expand and shift eastward or northward from its source region, guided by upper-level wind patterns.
When this hot, dry air moves, it brings characteristic weather conditions to regions like the Great Plains and the Mississippi Valley. The movement of cT air is associated with severe heat waves, clear skies, and extremely low humidity. The intense dryness and heat significantly increase the risk of drought and wildfires. This air mass stands in sharp contrast to the Maritime Tropical (mT) air from the Gulf of Mexico, which is warm and humid. The boundary between these two air masses is known as a dry line and is often a focus for severe weather.