Air mass

An air mass is a massive volume of air defined by its temperature and humidity. These volumes cover many hundreds or even thousands of square miles. As they move, they adapt to the characteristics of the surface below them. This process makes them a primary driver of global weather patterns. Because they are so large, an air mass can change the weather for entire regions.

Meteorologists use a specific classification system known as the Bergeron classification. This scheme uses three letters to describe an air mass. The first letter indicates moisture properties. A "c" stands for continental air masses, which are dry. An "m" stands for maritime air masses, which are moist. The second letter identifies the source region. "T" represents Tropical, "P" represents Polar, "A" stands for Arctic or Antarctic, "M" stands for monsoon, "E" stands for Equatorial, and "S" stands for adiabatically drying and warming air.

A third letter can describe the stability of the air mass. The letter "k" indicates the air is colder than the surface below it. The letter "w" means the air is warmer than the surface. For example, a polar air mass blowing over the Gulf Stream might be labeled "cPk." Scientists also use a degree tick or apostrophe to show that one air mass is colder than another of the same type. In some cases, notation like "mT/cP" shows the layering of different air masses over a region.

Tropical and equatorial air masses are hot because they develop at lower latitudes. Tropical air masses often have lower pressure because hot air rises. Continental tropical (cT) air masses are extremely hot and dry. They often originate in low-latitude deserts like the Sahara Desert. Other sources include the Arabian Peninsula and the Southwestern United States. Maritime tropical (mT) air masses are moist and are sometimes called trade air masses. These often affect the United States from the Caribbean Sea or the Gulf of Mexico.

Other types of air masses are much colder. Continental polar (cP) air masses are cold and dry. In North America, these often form over interior Canada. Arctic air is even deeper in its coldness than polar air. Arctic air develops over ice and snow-covered ground. Polar air masses develop over higher latitudes and are generally very stable. If maritime polar air moves over warmer ocean waters, it can lose that stability as it gains moisture.

When two air masses with different densities meet, they form a weather front. A front is a boundary that separates them. Fronts are the principal cause of most meteorological phenomena. Cold fronts may feature narrow bands of thunderstorms or severe weather. They often move from west to east and travel faster than warm fronts. Warm fronts are usually preceded by fog or steady precipitation. When a front becomes stationary, it can become a shearline. This is a line that separates regions with different wind velocities.

Air masses undergo modification when they move away from their source. Underlying vegetation, like a forest, can moisten an air mass through surface flux. Heat from warm water can also change an air mass very quickly. This can happen over distances as short as 100 kilometers. For instance, cold air moving across warm water can create lake-effect snow bands. This occurs when the temperature difference between the water and the air is larger than 13 °C (23 °F). Warmth and moisture rise from the water and condense into tall clouds.

These large-scale movements connect the atmosphere to the Earth's surface. The way air moves is influenced by mountains and large bodies of water. These features can slow down the movement of fronts. The depth of clouds and the rate of precipitation are directly affected by the water temperature. Understanding these connections helps scientists track how energy moves through our world. This study of air masses is essential for predicting the complex systems of our planet.