5 Air Masses That Determine U.S. Weather Systems

Rain clouds and rain in the Rocky Mountains over Colorado
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Other than clouds floating by, we don't often think about air moving overhead. But on a daily basis, huge bodies of air called air masses pass us by in the atmosphere above. An air mass is not only large (it can be thousands of miles across and thick), it has uniform temperature (hot or cold) and moisture (humid or dry) properties too.

As air masses are "pushed" around the globe by wind, they transport their warm, cool, humid, or dry conditions from place to place. It can take several days for an air mass to move over an area, which is why you might notice the weather in your forecast stays the same for several days on end, then changes and remains ​that way for several days, so on and so forth. Whenever you notice a change, you can attribute it to a new air mass moving over your region. 

Weather events (clouds, rain, storms) occur along the periphery of air masses, at boundaries called "fronts."

Air Mass Source Regions

To alter the weather conditions over the areas they traverse, air masses come from some of the hottest, coldest, driest, and wettest places on earth. Meteorologists call these air mass birthplaces "source regions." You can actually tell where an air mass is from by examining its name.

Depending on whether an air mass forms over an ocean or a land surface, it is called:

  • Maritime (m): Maritime air forms over oceans and other bodies of water and is humid. It is abbreviated by the lowercase letter m.
  • Continental (c): Continental air originates over land masses, and is therefore dry. It is abbreviated by the lowercase letter c.

The second part of an air mass name is taken from the latitude of its source region, which expresses its temperature. It is commonly abbreviated by a capital letter.

  • Polar (P): Polar air is cold and originates between 50 degrees N/S and 60 degrees N/S.
  • Arctic (A): Arctic air is extremely cold (so cold, it is sometimes mistaken for the Polar Vortex). It forms poleward of 60 degrees N/S.
  • Tropical (T): Tropical air is warm to hot. It forms at low latitudes, generally within 25 degrees of the equator.
  • Equatorial (E): Equatorial air is hot and originates along 0 degrees (the equator). Since the equator is mostly devoid of land areas, there is no such thing as continental equatorial air—only mE air exists. It rarely affects the U.S.

From these categories come the five combinations of air mass types that influence our U.S. and North American weather.

Continental Polar (cP) Air

Polar bear tracks in the snow-covered interiors of Canada and Alaska, where continental polar air forms

John E Marriott/All Canada Photos/Getty Images

Continental polar air is cold, dry, and stable. It forms over the snow-covered interiors of Canada and Alaska.

The most common example of continental polar air entering the U.S. comes in winter, when the jet stream dips southward, carrying cold, dry cP air, sometimes as far south as Florida. When it moves across the Great Lakes region, cP air can trigger lake effect snow.

Although cP air is cold, it also influences summer weather in the U.S. Summer cP air (which is still cool, but not as cold and dry as it is in winter) often brings relief from heat waves.

Continental Arctic (cA) Air

Continental arctic air forms over glacial landscapes like this Greenland ice-cap

Grant Dixon/Lonely Planet Images/Getty Images

Like continental polar air, continental arctic air is also cold and dry, but because it forms farther north over the Arctic basin and Greenland ice cap, its temperatures are generally colder. It is also generally only a wintertime air mass.

Does Maritime Arctic (mA) Air Exist?

Unlike the other North American air mass types, you won't see a maritime (m) classification for arctic air. While arctic air masses do form over the Arctic Ocean, this ocean surface pretty much remains ice-covered throughout the year. Because of this, even air masses that originate there tend to have the moisture characteristics of a cA air mass.

Maritime Polar (mP) Air

Lighthouse in Nova Scotia, where maritime polar air forms over oceans at high latitudes

Laszlo Podor/Moment/Getty Images

Maritime polar air masses are cool, moist, and unstable. Those affecting the U.S. originate over the North Pacific Ocean and the Northwestern Atlantic Ocean. Since ocean surface temperatures are typically higher than land, mP air can be thought of as milder than cP or cA air.

In winter, mP air is associated with nor'easters and generally gloomy days. In summer, it can lead to low stratus, fog, and periods of cool, comfortable temperatures.

Maritime Tropical (mT) Air

Tropical waters, beneath forming maritime tropical air
Fred Bahurlet/EyeEm/Getty Images

Maritime tropical air masses are warm and very humid. Those affecting the U.S. originate over the Gulf of Mexico, the Caribbean Sea, the western Atlantic, and the subtropical Pacific.

Maritime tropical air is unstable, which is why it's commonly associated with cumulus development and thunderstorm and shower activity. In winter, it can lead to advection fog (which develops as the warm, humid air is chilled and condenses as it moves over the cold land surface).

Continental Tropical (cT) Air

Continental tropical air forms over desert landscapes like in Nevada
Gary Weathers/Getty Images

Continental tropical air masses are hot and dry. Their air is carried from Mexico and the southwestern U.S., and only impacts U.S. weather during the summertime. 

While cT air is unstable, it tends to remain cloudless due to its extremely low humidity content. If a cT air mass lingers over a region for any period of time, a severe drought can occur.

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Means, Tiffany. "5 Air Masses That Determine U.S. Weather Systems." ThoughtCo, Aug. 27, 2020, thoughtco.com/types-of-north-american-air-masses-3443886. Means, Tiffany. (2020, August 27). 5 Air Masses That Determine U.S. Weather Systems. Retrieved from https://www.thoughtco.com/types-of-north-american-air-masses-3443886 Means, Tiffany. "5 Air Masses That Determine U.S. Weather Systems." ThoughtCo. https://www.thoughtco.com/types-of-north-american-air-masses-3443886 (accessed June 11, 2023).