Definition:
An air mass is a body of air whose physical properties, especially temperature, moisture content and lapse rate, are more or less uniform horizontally for hundreds of kilometres.
Once they move out of their SOURCE AREA, they become subject to modification, with the major changes occurring at the boundaries between air masses.
An air mass may be heated from below either by passing from a cold to a warm surface, or by solar heating of the ground over which the air mass is located. It can also therefore be cooled from below.
Heating from below acts to increase the instability of the air mass, so that the effect may be felt through a considerable thickness of air, whereas surface cooling often produces a temperature INVERSION, which limits the vertical extent of cooling.
One of the most important advances in meteorology was the realisation that many of the day to day changes in weather are associated with the formation and movement of boundaries between air masses: FRONTS.
The term front was developed by old favourites Bergeron, Bjerknes and Solberg, and the name was taken from the term used for armies meeting: the Western front etc.
It was observed that the typical geometry of the air mass interface takes on the form of a wave.
Similar wave patterns are found in other media e.g in sand when the tide goes out.
Frontal waves in the atmosphere are unstable i.e: they grow, and then dissipate. The cyclonic circulations are known as depressions, and are associated with converging air masses.