When a large body of air has fairly uniform characteristics it is referred to as an air mass. An air mass may be polar or tropical, which refers to its temperature. An air mass is also described as maritime or continental, in reference to its humidity.
The characteristics of an air mass are set when that air mass remains relatively still for a period of time, perhaps several days. For example, if an air mass originated from Canada, it would be cold and dry; a polar continental air mass. If an air mass became warm and moist as it moved over the tropical Pacific Ocean, it would be described as a tropical maritime. The areas from which the characteristics of an air mass are usually established are known as the source regions.
Once set in the source region, the characteristics of an air mass remain relatively set, but may be modified as the air mass passes over varying land and water features.
The place where two air masses meet each other is a front, and fronts bring weather.
Air Mass Stability
The weather inside a stable air mass will consist of smooth air and low visibility. Clouds tend to be of the stratiform type. If the air mass is unstable, high visibilities and turbulent conditions exist. Clouds tend to be of the cumuliform type.