The document discusses the causes of rainfall. It explains that air pressure is the force exerted by the weight of air. Differences in air temperature cause air pressure differences and air movement. There are three main types of rainfall: relief rainfall, which occurs when warm moist air rises over mountains and cools to form clouds; frontal rainfall, which happens when warm and cold air masses meet and the warm air is forced aloft; and convectional rainfall, caused by warm air rising from heated land and cooling to produce clouds and rain. Relief and frontal rainfall are the primary causes of rain in the UK.