An air mass is a large body of air with similar temperature and humidity characteristics that forms in stable source regions such as the subtropics or polar regions. There are five main air masses that affect Europe: tropical maritime (mild and moist), tropical continental (warm and dry), polar maritime (cool and moist), polar continental (cold and dry), and arctic maritime (cold and moist). Each air mass originates over a different source region and brings distinct weather conditions depending on its temperature and moisture content.