air mass

Large body of air with particular characteristics of temperature and humidity. An air mass forms when air rests over an area long enough to pick up the conditions of that area. When an air mass moves to another area it affects the weather of that area, but its own characteristics become modified in the process. For example, an air mass formed over the Sahara will be hot and dry, becoming cooler as it moves northwards. Air masses that meet form fronts.

There are four types of air mass. Tropical continental (Tc) air masses form over warm land and Tropical maritime (Tm) masses form over warm seas. Air masses that form over cold land are called Polar continental (Pc) and those forming over cold seas are called Polar or Arctic maritime (Pm or Am).

The weather of the UK is affected by a number of air masses which, having different characteristics, bring different weather conditions. For example, an Arctic air mass brings cold conditions whereas a Tropical air mass brings hot conditions.