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In the UK they are either polar or tropical, depending on where the air mass originated; they are also divided into maritime or continental, depending on whether the air has passed over land or sea. They come from all directions and can bring all types of weather.Source: BBC News
When they meet it creates a weather front. The air masses fight it out and the one that wins dictates the weather. There is a sixth air mass, the returning Polar Maritime, which is a variation of the Polar Maritime.