An air mass is a large mass of air which has similar temperature and humidity characteristics. This occurs generally when air is stationary for a period of time over one area. The air then takes on the physical features of that area. In the British Isles, we are affected by 5 air masses, which originate from colder, northerly areas or warmer, southerly areas. These five air masses are the Polar Maritime, Polar Continental, Arctic Maritime, Tropical Maritime and Tropic Continental.
The Polar Maritime is one of the most common air masses over the British Isles and is what causes the cool conditions throughout the year in most of Britain. The air mass warms slightly as it crosses the Atlantic Ocean to become unstable in the lower layers, which usually cases heavy rain showers, particularly as it crosses high land or mountains and is pushed upwards. These showers are also separated by bright sunny intervals. It is associated with cumuliform clouds and often strong winds and gales occur after the passing of this cold front.
The Polar Continental, on the other hand, is less common and results in much colder temperatures in winter, with many places reaching below 0oC.The air mass starts relatively stable, but warms slightly crossing the North Sea to become unstable in the lower layers and giving heavy snow fall, particularly in East Britain, whilst its clearer in the West. However, if this air stream occurs in summer it brings warm conditions and is more stable.
The Artic Maritime brings very cold conditions in winter, milder in spring and is very rare in summer. It slowly heats up as it crosses the sea, picking up some moisture and becoming unstable in its lower layers. It is the cause of the snow in winter in Scotland and the hail in spring, often in heavy showers. This air mass usually provides good visibility with little fog or precipitation and normally lasts several days. The air mass can cause temperatures in London to just be above freezing point in...