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An air mass is a large body of air that has similar temperature and moisture
properties. Air masses originate at source regions, which are large, usually
flat, areas of water or land, where air can be stagnant long enough to take on
the characteristics of the surface below. They move over long distances and the
bodies of water and land from which they started will affect the weather of the
areas to which they travel.
Air masses are defined according to their origin and the course they travel.
The names of the air masses hold clues as to their place of origin and,
consequently, the weather they will bring. Tropical air masses will bring warm
weather; Polar and Arctic masses will bring cold weather. Continental air masses
travel over land and will bring dry weather as they have not picked up much
moisture on the way. Maritime air masses travel over the sea so will bring wet
weather.
The temperature and humidity characteristics of the air masses affecting the
U.K. are shown in the diagram below:
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