Air masses and their sources

Air masses bring variety of weather. Six basic types of air masses affect the weather of the British Isles. They can bring anything from tropical warm and humid days to arctic cold depending on the type of air mass. Fronts form the boundaries of air masses with differing properties. The most severe weather usually occurs when dry-cold continental polar air clashes with warm-humid maritime tropical air.

The term 'air mass' was introduced some 70 years ago by Norwegian meteorologists from Bergen, Norway. Air mass is a large body of air, whose properties - temperature, humidity and lapse rate - are largely homogeneous over an area several hundred kilometres across.

The nature of air masses is determined by three factors: the source region, the ageand the modifications that may occur as they move away from their source region across the earth's surface.

The primary classification of air masses is based on the characteristics of the source region, giving Arctic (A), Polar (P) or Tropical air (T), and on the nature of the surface in the source region: continental (c) or maritime (m). In addition, a large variety of secondary types of air masses are defined. For example, equatorial air (E) or Mediterranean air. Sometimes there is a letter (k) or (w) attached to the two-letter initials indicating whether the air is wa rmer or colder than the surface. The former becomes more stable, and the latter more unstable.

Some older works use the term of an 'returning air mass'. This usually refers to maritime polar air that has been altered moving across the relatively mild Atlantic and is returning polewards eventually.

http://www.weatheronline.co.uk/reports/wxfacts/Air-masses-and-their-sources.htm