Air Masses and Air Streams
Air in the atmosphere picks up the characteristics of the area it comes from. For example, the air above the Sahara Desert tends to be hot and dry. A large volume of air that has similar temperature and humidity is called an air mass.
If an air mass moves from its source region it will bring its "weather" with it. So air that starts over the Sahara stays hot and dry for a while and brings hot and dry weather. The name given to this body of moving air is an air stream.
There are five air streams that affect Britain. They are shown in the table below.
| Name | Source Area | Weather |
| Tropical Maritime (mT) | The Atlantic Ocean, south-west of Britain | Warm and wet |
| Tropical Continental (cT) | South Europe and North Africa, including the Sahara Desert | Warm and dry |
| Polar Maritime (mP) | The North Atlantic | Cold and wet |
| Polar continental (cP) | Northern Europe and Siberia | Dry, and in winter, very cold |
| Arctic maritime (mA) | Arctic Ocean | Very cold, but not very wet due to the low humidity |
The map below shows these air streams.
