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An air mass is a large body of air that has consistent temperature and moisture content throughout. These characteristics are determined by the region over which the air mass forms, known as its source region. Air masses can cover thousands of square kilometers and extend vertically through much of the atmosphere. When air masses move away from their source regions, they retain their original temperature and moisture properties for a considerable time.

The classification of air masses is based on their latitude and their moisture content. Latitude is divided into polar (P) for cold air masses and tropical (T) for warm air masses. Moisture content is divided into continental (c) for dry air masses and maritime (m) for moist air masses. This results in four basic types of air masses: cP (continental polar), cT (continental tropical), mP (maritime polar), and mT (maritime tropical). Less common types like arctic (A) and equatorial (E) also exist.

The interaction and collision of different air masses are responsible for most of Earth's weather. When two air masses with different properties meet, a boundary called a front forms. The type of front and the characteristics of the colliding air masses determine the kind of weather that develops, such as precipitation, storms, and changes in temperature and wind. The movement and behavior of air masses are a fundamental aspect of meteorology.