An air-mass is a large body of air whose temperature and relative humidity values more or less remains uniform in a given horizontal plane with dimensions of about 500nm or so.
The idea of the air-masses was first introduced by a Norwegian scientist called Bergeron in 1928.Because the temperature of the air depends almost entirely on contact with the earth’s surface, the air over Polar Regions will be cold, while that over Tropics will be warm. Airmasses are named by the sources from which they originate and hence the main air-masses are Polar air-masses and Tropical air-masses or Arctic (Antarctic airmasses). They are further sub-divided as maritime (high humidity) or continental (low humidity) as in the following table:
