The water cycle

Water is taken up as vapour into the atmosphere from the Earth’s surface, from the seas, lakes, rivers, soils, and from plants and animals. It is then transported to various heights and sometimes over great distances with the air. After some time the vapour condenses again, forming ice crystals or water droplets, and thereby producing mists, fogs and all the different kinds of clouds (whose form depends on the air motion, the water content and the nature and sizes of the particles they carry). Sooner or later the H₂O molecules are put down once more on the Earth’s surface: as rain, snow, hail, etc., or in a deposit of dew or ice. The average sojourn of a water molecule in the atmosphere is reckoned at about 10 days, though the figure covers an enormous range of variation. Some molecules get carried up into the stratosphere in which they are liable to stay for from 7 to 10 years. At the other extreme, some molecules evaporated from wet or sodden ground may be soon drawn into the lower layers of a thunderstorm cloud and precipitated again within the hour. The overall average itself probably varies with the vigour of the general atmospheric circulation. During their time in the atmosphere the water molecules are the stuff from which most kinds of weather are made. Once redeposited the water may spend weeks, months or years on its way to the sea, and as we saw in Chapter 8 some of it spends thousands of years in the deep ocean before being presented to the atmosphere again. Some water becomes trapped in the subsoil and in porous strata of rock for even longer periods: a radiocarbon age of 25 000 years has been measured in oases in the Sahara in water that has passed through the strata below the desert. And some of the ice in the depths of the Greenland and Antarctic ice sheets has been embedded for hundreds of thousands of years.