An air mass is a large body of air which has similar characteristics of temperature, humidity and pressure. During the course of a year, Ireland can come under the influence of different air masses, including Polar Maritime, Arctic Maritime, Polar Continental, Tropical Continental and Tropical Maritime. Each air mass results in different weather types being experienced (Fig. 14.1).
Arctic Maritime Polar Maritime
Polar Continental
Tropical Maritime Tropical Continental
Fig. 14.1 Air masses that influence Ireland's weather.
Depression
A depression is an area of low pressure which forms in the North Atlantic along the polar front. This is the boundary between warm, moist air moving northwards and colder, denser, drier air moving southwards. Composed of three different sectors, depressions generally move from west to east over Ireland; their overall wind direction is anticlockwise (Fig. 14.2).
