The same water, over and over
The Earth has had roughly the same amount of WATER for billions of years, going round and round the same CYCLE. The Sun warms the OCEAN, a LAKE, a RIVER or a PUDDLE, and some of the water turns into invisible VAPOUR and rises: that is how it comes to EVAPORATE. High up, where the air is colder, the vapour cools and the droplets CONDENSE into a CLOUD, then fall again as RAIN or SNOW. The water in your glass has almost certainly been through clouds, rivers, seas and living things countless times.
Plants join in too. Trees pull water up from their roots and let it out through tiny holes in their leaves, and a large oak can give off hundreds of litres on a hot summer day. Our nature page covers how the whole outdoors is linked by this flow.
Forests can even make their own rain. The trees of the Amazon give off so much water vapour that moist air blowing across South America rains down, rises from the trees again and rains again further on, sometimes called a flying river. Cutting down large areas of forest can break that chain and dry out regions far away.
A cloud weighs as much as a hundred elephants
Clouds look weightless, but a fluffy summer cumulus cloud of ordinary size holds around 500 tonnes of water, roughly the weight of a hundred elephants. It floats because that water is spread out as trillions of tiny droplets, each far too small and light to fall, held up by rising warm air. Only when droplets bump together and grow heavy enough do they fall as rain. Snowflakes drift down far more slowly than raindrops, and one falling from a high cloud can take the best part of an hour to reach the ground.
Raindrops are not shaped like teardrops, either. Small ones are round, and bigger ones are squashed flat on the bottom by the air rushing past, more like the top half of a burger bun. The smell after rain on dry ground even has a name, petrichor, coined by two Australian scientists in 1964; part of it comes from a chemical made by bacteria in the soil, which the human nose can detect in tiny amounts.
How long water stays put
A molecule of water that evaporates spends, on average, only about nine days in the air before it falls again. A molecule that reaches the deep ocean may stay there for thousands of years, and one locked in the ice of Antarctica for far longer. Some water soaks deep underground and can lie there for tens of thousands of years.
Only a tiny share of the Earth's water is fresh and easy to reach. About 97 per cent is salty sea water, and most of the fresh water is frozen in ice caps and glaciers. Rain that flows over the ground into streams is called RUNOFF, and the gathering of water in rivers, lakes and seas is COLLECTION. FREEZING and MELTING, MIST and MOISTURE, a HUMID day, a DROPLET, SUNSHINE and a STORM all belong to the cycle, and our weather page follows what happens when the water comes back down.