An air mass is an extensive region of air that possesses similar moisture and temperature readings. Air masses form when air sits still in a region for several days or weeks, according to meteorologist Jeff Haby. When the upper level winds of the jet stream encounter the air mass, they guide cooler masses to the lower latitudes and warmer masses towards the higher latitudes.
As the air mass moves, it acquires the characteristics of the area it covers, either warming up or cooling off, for instance, before moving again in response to the upper level winds. An air mass may modify several times over the course of months.Learn more about Earth Science