An air mass is a large body of air that has similar temperature and moisture properties throughout. The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below. Air masses are defined according to both their region of origin and their course travelled. The main types include maritime tropical (mT), maritime polar (mP), maritime Arctic (mA), continental polar (cP) and continental tropical (cT) air masses. These descriptions are used both in the Britain and Europe, and in the United States. The maritime Arctic (mA) air mass, for example, originates in the Arctic and travels across the relatively warm stretch of the North Sea to affect the climates of northern Europe. Maritime tropical air (mT), on the other hand, originates near the Gulf of Mexico and travels north-east across the warm Atlantic to affect western Europe, as well as north-west across the United States. In Europe, continental polar air comes from Siberia, whilst in North America, it originates over the northern plains of Canada.
As a general rule, maritime air masses are generally fairly moist, containing considerable amounts of water vapour, which is ultimately condensed and released as rain or snow. By contrast, continental air is usually a lot drier. Once an air mass moves out of its source region, it may be modified as it encounters surface conditions different to those found in the source region. For example, as polar air moves southward, it encounters warmer land masses that heat it by the ground below. Polar and tropical air masses typically clash in the middle latitudes, producing some very changeable climates, typically experienced in the British Isles.