So, how is an 'air mass' defined?
An air mass is classically defined as a large body of air (many hundreds to a few thousands of km in extent),having quasi-uniform horizontal temperature and humidity characteristics. Indeed, once upper-air soundings became available on a regular basis, it could be seen that this uniformity extended vertically, such that each air mass has a distinct vertical profile of temperature and humidity.
To attain these uniform (or nearly so - nothing is that clean-cut in meteorology!) signatures, a large body of air has to remain over one area for a considerable time - measured in weeks rather than days. This requires a pressure pattern which allows stagnation of the air - and this usually means a slow-moving anticyclone such as is found in the great sub-tropical high pressure belts, the polar high pressure regions or the Asiatic (or other great continental) winter anticyclones. These are said to be the 'source' regions of an air mass. Once an air mass leaves its source region, it is modified, depending largely upon the type and temperature of the underlying surface over which it moves.
For example, air that moves polewards from the sub-tropical high pressure belts encircling the earth will be cooled from below as it passes over progressively colder seas, and this will in turn affect the relative humidity (increasing it leading to formation of cloud/precipitation), and although these processes may slightly lower the absolute humidity, it will still have a higher humidity value than air coming from polar latitudes, which will be warmed from below and will become increasingly buoyant as heat is input to the lower layers.
Air Masses can be classified as 'polar' (having originated in cold/high latitude regions), or 'tropical' (having come out of the stagnant regions around the sub-tropical high. They are further sub-classified as either 'maritime': having passed over a sea surface, or 'continental' having moved over a land mass. This then gives rise to the four principal types of interest to us in north-west Europe:- tropical maritime (Tm or mT), polar maritime (Pm or mP), tropical continental (Tc or cT) and polar continental (Pc or cP). There are of course many modifications , and a full treatment of air masses is outside the scope of this FAQ. See the list of recommended reading.