Water that transfers from the atmosphere to the Earth's surface is called precipitation. It may be in the form of rain, snow or hail. Water vapour may also precipitate by condensing as dew or hoar frost. Water in the atmosphere, although one of the smallest reservoirs, is the most important reservoir of water. It exists as vapour, liquid (clouds and raindrops) or in solid form (snow and ice). When air rises (as it does when it moves over mountains), it expands, owing to the decrease in pressure with altitude, and as it expands it cools at a rate of around 1 °C per 100 metres of altitude. As the air cools, water vapour condenses around small particles suspended in the air, such as pollen grains, fungal spores, dust and salt from sea spray. This condensation results in the formation of clouds composed of 0.001-0.1 mm diameter droplets. Precipitation of this water occurs when these droplets coalesce to form larger drops about 1 mm in diameter, or when ice crystals form and they fall to the Earth's surface.
The net effect of the water cycle is to transfer some 36 × 1015 kg of fresh water each year from the oceans to the land by precipitation (Figure 2.2). Globally, precipitation is distributed very unevenly — both geographically and seasonally. Figure 2.4 shows the global distribution of precipitation on land. Some areas get less than 250 m mannually (precipitation is usually measured in millimetres); these are usually deserts, such as in North Africa, the Middle East, central Asia and central Australia. Some of these are visible as the yellow/buff areas in Figure 2.1. The annual rainfall in other areas may reach as much as 12 000 mm; this heavy rainfall is characteristic of the Amazon Basin and parts of South and South-East Asia. The greatest recorded rainfall in 24 hours was 1870 mm on Reunion Island, Indian Ocean, in 1952.
Precipitation can vary in amount from year to year, and in many regions it is seasonal, as on the Indian subcontinent where the south-west monsoon brings rain for a few months in the summer only. Irregular rainfall is common in drier areas, where rain may fall for only a few days each year; the equivalent of a year's rain may fall in one storm lasting a few hours.
The yearly irregularity of rainfall in some areas can be seen in Figure 2.5, which gives the precipitation in Niamey, the capital of Niger, in sub-Saharan Africa (the Sahel). Areas such as the Sahel, with a fairly low rainfall and high variability, are classed as semi-arid, and are vulnerable to drought in years of lower than average rainfall. Another feature of the precipitation in the Sahel is the seasonality of the rain — Niamey averages only 44 rainy days a year, confined to a rainy season (Figure 2.6).
Because of the prevailing westerly winds (due to its position in mid-latitudes), the UK's precipitation is more evenly distributed throughout the year (Figures 2.8 and 2.9) and more constant from year to year than in many other countries. The amount of precipitation does, however, vary from place to place (Figures 2.9 and 2.10). In the east of England the average annual precipitation is about 500 mm, but in mountainous parts of north-west Scotland, the Lake District and North Wales it can reach over 3200 mm (Figure 2.10).
Range (as seen in Figure 2.9):
England 55 - 90mm
Scotland 78 - 170mm
Wales 82 - 173mm
Northern Ireland 68 - 118mm
- England has the lowest range, Scotland and Wales the highest.
Altitude Precipitation increases with altitude over upland areas. When air rises, it cools, causing precipitation, and this occurs when the prevailing westerly airstreams are deflected upward over coastal mountains in Scotland, the Lake District and Wales, the wettest parts of mainland UK.
Low-pressure weather systems Bands of rain are associated with the passage of warm and cold fronts mainly west to east across the UK.
Heating of the ground surface Heating of the ground surface causes convective heating of the atmosphere, giving local showers and thunderstorms.
The regional variation is caused by differences in altitude and in the paths of low-pressure weather systems crossing the UK. More weather systems pass across the north of the UK than the south, so the north is wetter. This creates an uneven distribution of precipitation in Britain, especially in relation to population density: the sparsely inhabited mountainous areas, with the lowest demands for water, have the highest precipitation.