An air mass is a vast body of air covering thousands of square miles with relatively uniform characteristics of temperature and humidity. These bodies acquire traits by remaining stationary over a source region for an extended period, adapting to the surface below. Air masses are classified by their latitude of origin (Polar, Arctic, or Tropical) and whether they form over land (continental) or water (maritime), which dictates moisture content. The movement of these masses drives daily weather patterns, and this article focuses on the Continental Tropical (cT) air mass.
Defining Continental Tropical Air Masses
Continental Tropical air masses are designated by the letter combination ‘cT,’ signifying their formation over land and within tropical or subtropical latitudes. The “Continental” descriptor means the air mass originates over a landmass, which inherently leads to dry conditions because land surfaces lack the moisture source of oceans. The “Tropical” designation places their source regions generally between 20° and 35° latitude, where solar radiation is intense.
These air masses primarily form over large, arid land areas, particularly deserts and high plateaus. Significant source regions include the Sahara Desert in North Africa, the Arabian Peninsula, and the desert southwest of the United States and northern Mexico. Continental Tropical air masses are most prevalent and well-developed during the summer months when intense solar heating of the land maximizes their thermal properties.
Core Thermal and Moisture Properties
Continental Tropical air masses are defined by conditions of extreme heat and remarkable dryness. Their formation over sun-baked desert regions causes the air to absorb substantial thermal energy, resulting in very hot temperatures. This intense heating of the lower atmosphere often leads to a high lapse rate near the surface, meaning the temperature drops rapidly with increasing altitude.
The defining lack of moisture is due to two main factors: the arid nature of the source region and atmospheric dynamics. Since the air mass forms over dry land, it has very low specific humidity (the actual amount of water vapor in the air). Furthermore, these regions are often dominated by high-pressure systems where air sinks, a process called subsidence. This sinking air warms adiabatically, further reducing its relative humidity and making the air extremely dry.
Weather Impacts and Geographic Scope
The movement of Continental Tropical air masses typically results in scorching heat waves and prolonged dry spells in affected regions. Because the air is so dry, it inhibits the formation of clouds and precipitation, leading to persistent clear skies and maximum solar insolation. This lack of cloud cover allows for extreme daytime heating, and rapid nighttime cooling due to unimpeded outgoing longwave radiation.
The combination of intense heat and dry air results in large diurnal temperature ranges, where the difference between daytime highs and nighttime lows is significant. When these air masses move poleward, such as into the Great Plains and Midwest of North America during the summer, they can cause severe drought conditions and record high temperatures. Although they rarely reach the eastern or southeastern United States, their influence can occasionally be felt, bringing intense, dry heat before gradually acquiring moisture and moderating their characteristics as they travel over different surfaces.