An air mass usually forms in an area of low pressure because it's stable low pressure because it's unstable high pressure because it's stable high pressure because it's unstable

high pressure because it's stable

Explanation:

Air mass is volume of air which has stable temperature, humidity and pressure horizontally. Over time, each air mass acquires properties of the region by residing over same part of a surface.

Areas of low pressure and high pressure occur where there is warm air and cold air respectively. An air mass usually forms over an area of high pressure. Warm air rises up and cold air takes its place. Warm air has low density and low pressure where as cold air has high density and pressure and therefore, sinks to the bottom. This is a stable condition.  The movement of air mass is responsible for maintenance of temperature conditions on Earth.