Air masses form when a large body of air has similar temperature and humidity characteristics throughout. They develop when an air mass stays in one place for a while, allowing it to take on the characteristics of the surface below it. For example, an air mass that forms over a warm, moist ocean will be warm and moist, while an air mass that forms over a cold, dry desert will be cold and dry.

Air masses can also form when a front, which is a boundary between two different air masses, stalls or becomes stationary. When this happens, the air masses on either side of the front can become separated and isolated from each other, leading to the formation of two distinct air masses.

The formation of air masses is important because they influence the weather and climate of a region. For example, a warm, moist air mass can bring warm, rainy weather, while a cold, dry air mass can bring cold, dry weather. The movement of air masses can also lead to the formation of storms, such as hurricanes and blizzards.