An air mass is a large volume of air in the atmosphere that is mostly uniform in temperature and moisture. Air masses can extend thousands of kilometers across the surface of the Earth, and can reach from ground level to the stratosphere—16 kilometers (10 miles) into the atmosphere.

Air masses form over large surfaces with uniform temperatures and humidity, called source regions. Low wind speeds let air remain stationary long enough to take on the features of the source region, such as heat or cold. When winds move air masses, they carry their weather conditions (heat or cold, dry or moist) from the source region to a new region. When the air mass reaches a new region, it might clash with another air mass that has a different temperature and humidity. This can create a severe storm.

Meteorologists identify air masses according to where they form over the Earth. There are four categories for air masses: arctic, tropical, polar and equatorial. Arctic air masses form in the Arctic region and are very cold. Tropical air masses form in low-latitude areas and are moderately warm. Polar air masses take shape in high-latitude regions and are cold. Equatorial air masses develop near the Equator, and are warm.

Air masses are also identified based on whether they form over land or over water. Maritime air masses form over water and are humid. Continental air masses form over land and are dry.

Therefore, an air mass that develops over northern Canada is called a continental polar air mass and is cold and dry. One that forms over the Indian Ocean is called a maritime tropical air mass and is warm and humid.

Air masses are classified on weather maps using two or three letters.

  • A lowercase letter describes the amount of moisture in the air mass: m for maritime (moist) and c for continental (dry).
  • An uppercase letter describes the heat of the air mass: E for equatorial, T for tropical, M for monsoon, P for polar, A for Arctic or Antarctic, and S for superior—a unique situation with dry air formed by a powerful downward motion of the atmosphere.
  • A lowercase letter describes the relationship between the air mass and the earth: k signifies that the air mass is colder than the ground below it, while w describes an air mass that is warmer than the ground below it.
air mass
Air masses are separated from each other by boundaries called fronts.
Noun

layer of gases surrounding Earth.

Noun

a large volume of air that is mostly consistent, horizontally, in temperature and humidity.

Arctic air mass
Noun

large volume of cold air that forms over the Arctic and Antarctic regions.

Noun

layers of gases surrounding a planet or other celestial body.

continental air mass
Noun

large volume of dry air that forms over land.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

equatorial air mass
Noun

large volume of air that forms near the Equator.

Noun

amount of water vapor in the air.

maritime air mass
Noun

large volume of air that forms over the ocean.

meteorologist
Noun

person who studies patterns and changes in Earth's atmosphere.

moisture
Noun

wetness.

polar air mass
Noun

large volume of air that forms in high-latitude regions.

source region
Noun

surface over which an air mass forms.

stationary
Adjective

unmoving.

storm
Noun

severe weather indicating a disturbed state of the atmosphere resulting from uplifted air.

stratosphere
Noun

level of Earth's atmosphere, extending from 10 kilometers (6 miles) to 50 kilometers (31 miles) above the surface of the Earth.

Noun

degree of hotness or coldness measured by a thermometer with a numerical scale.

tropical air mass
Noun

large volume of air that forms in low-latitude regions.

uniform
Adjective

exactly the same in some way.

volume
Noun

space an object occupies.

Noun

state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.

Noun

movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.