Precipitation - rain, snow, sleet and hail - is associated with areas of rising air and low pressure. When air rises it cools, and the moisture it contains condenses out as clouds, which eventually produce precipitation. In regions of high pressure, air is descending, the atmosphere is stable, the skies are usually clear, and precipitation is rare.
The highest rainfall totals occur near the equator in the tropics, where the strong heating by the Sun creates significant vertical uplift of air, and the formation of prolonged heavy showers and frequent thunderstorms. Annual rainfall totals in the tropics usually exceed 100 inches or 2,500 millimetres, and can be as high as 400 inches or 10,000 millimetres, particularly if influenced by the monsoons or if mountains enhance the uplift of air.
Within the polar regions precipitation is low because air is too cold to contain much water vapour. In addition, the cold heavy air descends precluding much cloud formation. In fact, some parts of Antarctica and the Arctic are as dry as the hot desert climates of the subtropics, where high pressure also limits cloud formation and precipitation. Both hot and cold deserts may receive less than 10 inches or 250 millimetres of precipitation each year. Indeed, in some parts of the subtropics, rain may not fall for several years.
The temperate mid latitudes have moderate levels of precipitation, much of it associated with the development of frontal depressions which form when warm subtropical and cold polar air masses collide. On western facing coastlines, annual precipitation may approach 100 inches or 2,500 millimetres. This diminishes substantially within the interiors of major landmasses, particularly the continents of North America and Asia, as the air dries and moves further away from its ocean source of moisture.
Like temperature, patterns of rainfall shift with the seasons, and the north-south movement of the Sun, particularly within and near the tropical rain belt. Regions closest to the equator may experience two wet seasons and two dry seasons. Greatest rainfall occurs at the March and September Equinoxes, when the midday Sun over the equator is directly overhead and is at it strongest. Away from the equator, the year may be split into single wet and dry seasons. In the Northern Hemisphere, heaviest rain occurs in June and July, and may be augmented by the monsoons. In the Southern Hemisphere, the wet season occurs in December and January.
In the temperate mid-latitudes, frontal low-pressure systems dominate in winter, and rainfall tends to be higher at this time of year, as in the UK. During the summer, the subtropical high-pressure anticyclones expand to influence the weather patterns. Consequently, precipitation is somewhat lower, and most which falls is convective in nature, arising from the uplift of air during elevated daytime heating and the generation of heavy short-lived showers and thunderstorms.