The Endless Journey of the Water Cycle

The water that fills a glass on the table may seem to have a very ordinary history, but it does not. The same water has fallen as rain, flowed in rivers, floated as cloud and lain frozen in ice countless times over, for the Earth makes no new water and loses almost none. Instead the water it already has is used again and again, moving endlessly between the sea, the sky and the land in a great loop that scientists call the water cycle. Understanding this cycle explains where rain comes from, why rivers keep flowing and how the whole living world is kept supplied with the water it needs.

The engine that drives the entire cycle is the Sun. Its heat warms the surface of the oceans, lakes and rivers, and this warmth causes water at the surface to change from a liquid into an invisible gas called water vapour. This process, in which liquid water becomes a gas and rises into the air, is known as evaporation. Vast amounts of water leave the sea in this way every day, silently and without our noticing, and it is the Sun alone that provides all the energy needed to lift it.

As the warm, moist air rises, it carries the water vapour higher into the sky, where the air is much colder. When the vapour is cooled enough, it changes back from a gas into tiny droplets of liquid water, a process known as condensation. These droplets are so small and light that they float, and when countless millions of them gather together they form the clouds we see drifting overhead. A cloud, for all its size, is nothing more than a great crowd of these minute water droplets suspended in the air.

Inside a growing cloud the droplets bump into one another and join together, and as they do so they grow larger and heavier. Eventually they become too heavy for the air to hold up, and they fall back to the ground. This falling water is called precipitation, a single word that covers rain, snow, sleet and hail alike, the form it takes depending chiefly on how cold the air is on the way down. In this way the water that the Sun lifted from the sea is returned once more to the surface of the land and the ocean.

What happens to the water after it lands completes the loop. Some of it is pulled downhill by gravity, gathering into streams and rivers that flow steadily back towards the sea. Some soaks down into the soil and collects deep underground, where it is stored as what is called groundwater and may slowly feed springs and wells for years to come. Some is taken up by the roots of plants, and some simply evaporates straight back into the air. Sooner or later, however, a large part of it finds its way back to the ocean, ready to begin the journey all over again.

Because the cycle turns endlessly, the total amount of water on the planet stays very nearly the same from one age to the next; it is neither created nor destroyed, only borrowed and passed on. The very water we drink today has been travelling this circuit for billions of years and has, in its time, been part of ancient oceans, distant storms and long-vanished rivers. Every drop, in a sense, is on loan from a cycle far older than humankind, and it will move on again long after we have finished with it.

The water cycle is more than an elegant piece of natural machinery, for all life on land depends upon it. Without the constant lifting of water from the sea and its return as rain, the continents would be barren, the rivers would run dry and nothing could grow. The quiet, ceaseless turning of this great wheel is what keeps the land green and the living world supplied, and it goes on above and around us at every moment, whether or not anyone happens to be watching.