Air Masses

An air mass is a large mass of air that has similar characteristics of temperature and humidity within it. An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days, or longer, it picks up the distinct temperature and humidity characteristics of that region.

Air Mass Formation

Source regions of common air masses. The arctic front is along the northern border of Alaska and Canada. The polar front is along the northern border of the United States.Where an air mass receives it's characteristics of temperature and humidity is called the source region. Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it. Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms. How would a moving air mass form an inversion? When a warmer air mass travels over colder ground, the bottom layer of air cools and, because of its high density, is trapped near the ground.

In general, cold air masses tend to flow toward the equator and warm air masses tend to flow toward the poles. This brings heat to cold areas and cools down areas that are warm. It is one of the many processes that act towards balancing out the planet’s temperatures. Air masses are classified based on their temperature and humidity characteristics. Below are examples of how air masses are classified over North America.

  • Maritime tropical (mT): moist, warm air mass
  • Continental tropical (cT): dry, warm air mass
  • Maritime polar (mP): moist, cold air mass
  • Continental polar (cP): dry, cold air mass

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