Case Studies

Case study 1 Tropical Maritime airmass

This is probably the most common air mass that affects the British Isles, and it can do so at any time of the year, reaching it from near the Azores in the mid Atlantic Ocean, with low pressure to the north west and high pressure lying over France. Winds are south westerly and may be quite strong in winter. Having formed over relatively warm waters, it is also relatively mild or warm and very moist, at least in its lowest layers. As it approaches south-western and western parts of the British Isles it usually gives a lot of cloudy, damp weather with sea and hill fog. Although in winter it often stays dull and cloudy at low levels as the air mass crosses the country, in the summer insolation may be strong enough to dry out the air and warm it up quite substantially, giving some quite pleasant sunny weather by day in the eastern parts of both England and Scotland.

Case study 2 Polar Continental airmass

This is a much less common event and generally occurs in wintertime (say from December-April) with high pressure lying across Scandinavia and low pressure over France. The winds reach the British Isles from an east or north easterly direction. Although the air mass begins its journey cold and dry, after stagnating over the snow covered cold interior of northern Europe, as it crosses the North Sea it may pick up enough moisture to allow light snow showers or freezing drizzle to fall, as it lifts up over the North Sea coasts. Further south and west it dries out again and gives generally cold, dry sunny weather with frosts by night. Further west again as it crosses the Irish Sea although the temperatures will rise, it may pick up further moisture again and allow it to give a few sleety showers over the coasts of Ireland as well.