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.