• 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.
  • Term Part of Speech Definition Encyclopedic Entry
    air Noun

    layer of gases surrounding Earth.

    Encyclopedic Entry: air
    air mass Noun

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

    Encyclopedic Entry: air mass
    Arctic air mass Noun

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

    atmosphere Noun

    layers of gases surrounding a planet or other celestial body.

    Encyclopedic Entry: atmosphere
    continental air mass Noun

    large volume of dry air that forms over land.

    Equator Noun

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

    Encyclopedic Entry: equator
    equatorial air mass Noun

    large volume of air that forms near the Equator.

    humidity Noun

    amount of water vapor in the air.

    Encyclopedic Entry: humidity
    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


    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


    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.

    temperature Noun

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

    Encyclopedic Entry: temperature
    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.

    weather Noun

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

    Encyclopedic Entry: weather
    wind 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.