​What is an Air Mass?

​​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.

​Where do Air Masses Form?

​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.

​Types of Air Masses

​Meteorologists identify air masses according to where they form over the Earth. There are two main categories for air masses: tropical and polar.

Polar air masses form in the arctic region and are very cold. Tropical air masses form in low-latitude areas and are moderately 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.

​Air Masses

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 and c for continental).

• ​An uppercase letter describes the heat of the air mass: T for tropical and P for polar.

• ​There are 4 air mass abbreviations, which can be seen in the diagram to the right.

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