An air mass is a batch of air that has nearly the same temperature and humidity. An air mass acquires these characteristics above an area of land or water known as its source region. When the air mass sits over a region for several days or longer, it picks up the distinct temperature and humidity characteristics of that region.
Air Mass Movement
Air masses are slowly pushed along by high-level winds. When an air mass moves over a new region, it shares its temperature and humidity with that region. So the temperature and humidity of a particular location depends partly on the characteristics of the air mass that sits over it.
Storms arise if the air mass and the region it moves over have different characteristics. For example, when a colder air mass moves over warmer ground, the bottom layer of air is heated. That air rises, forming clouds, rain, and sometimes thunderstorms.
An air mass has roughly the same temperature and humidity.
Air masses form over regions where the air is stable for a long enough time that the air can take on the characteristics of the region.
Air masses move when they are pushed by high level winds.