When an air mass remains stationary over an area for several days it tends to assume the temperature and humidity of that area. Stationary air is most commonly found in high pressure belts in the sub-tropics and high latitiudes. See Pressure and Winds. The areas in which these homogeneous air masses form are called source regions. Air masses are classified according to the region they originated from and the surface over which they formed: Arctic (A), very cold air; Polar (P),cold air; Tropical (T), warm air; continental (c),dry air; Maritime (m), moist air. There are six major air masses which affect the British Isles (Am, Pc, Pm, Tm, Tc, RPm [Returning Polar maritime] )

Tropical Maritime:
This air mass usually approaches Britian from the south-west, comming from the Azores or occasionally the Caribbean. But, surprisingly it heralds dull, clammy and overcast weather. This is because it is very hot and moist when it leaves, but as it travels over the ocean it cools down yet retains its moisture. So it reaches Britain saturated (with dew points are often in the region of 15 C) giving mild wet weather in winter and warm, though not hot, weather in summer. The lower air is stable but if it is forced to rise over hills it can become unstable and result in thundery showers.

Tropical Continental:

This air mass is typically hot and dry and comes from the south or south-east. Originating in the Sahara and then crossing France, Spain or the Mediterranean it is not surprising that this air mass is hot. It picks up a certain amount of moisture in crossing the mediterranean and the Channel, but dew points are still fairly low (about 8C). The majority of tropical continental airstreams give us a marvellous heat wave such as 1989 and 1990. We rarely find tropical continental air masses arriving in Britain in the winter months.

Polar Continental:
This air mass usually originates in Scandinavia or Russia, arriving here from the east or north-east. Typically Polar Continental air is cold and dry. The air is particularly dry when it has only travelled a short distance across the channel (eg Calais-Dover) but is more moist if it has crossed the North sea before reaching Britian. Dew points in this air mass are low, often being below freezing. In winter polar continental gives very cold temperatures. It begins its journey very stable but warms slightly crossing the sea and becomes unstable in lower layers giving snow or hail on reaching the eastern coast of Britain. In summer the air stream is warmer (as Siberia is hotter during the summer) and it brings warm stable conditions.

Polar Maritime:
This is one of the most common airstreams to reach Britain. This air mass usually arrives from a westerly direction, originating in Canada or Greenland and is cool and fairly moist (although unable to carry a great deal of moisture because of its temperature). It brings with it cool conditions throughout the year. The very cold polar air warms slightly as it crosses the Atlantic, becoming ustable in its lower layers. This means clouds and showers along the west coast in winter as the east is sheltered by hills and mountains. But in the summer the hot land triggers convective clouds and the western mountain shelter is less important and showers and thunderstorms are found throughtout the country.

Arctic Maritime:
This northerly air mass gives very cold conditions in winter and spring and is rare in summer. It heats up as it crosses the sea, becoming unstable and causing snow and strong winds (hails in spring) as it hits the Northern edge of Scotland and Ireland and there will be very cold temperatures in the rest of the country (often below freezing in London). This weather often lasts several days.

Returning Polar Maritime:

This air mass originates in North Canada as does Pm, but moves further south over the ocean and therfore warms up more, becoming unstable, warm and moist. In summer this airstream heralds very warm, cloudy, showery weather. In winter it gives cloudy weather with showers in the west.