How the Water Cycle Moves Water Around Earth

Most of Earth's water is stored in the oceans. Sunlight warms the surface and gives some water molecules enough energy to escape as water vapour. This change from liquid to gas is called evaporation.

Plants add water vapour through transpiration. They take in water through their roots, move it through their tissues and release some through tiny openings in their leaves. Scientists often group evaporation and transpiration together because both move water from the surface into the atmosphere.

Water also evaporates from lakes, rivers, soil and wet surfaces. The rate changes with temperature, wind, humidity and the amount of exposed water. A warm, windy and dry day usually supports faster evaporation than a cool, still and humid one.

Water vapour is invisible. As moist air rises and cools, some vapour changes back into tiny liquid droplets or ice crystals. This process is condensation. Large collections of those droplets and crystals form clouds.

Cloud particles are very small and can remain suspended in moving air. As they collide and combine, they may grow heavy enough to fall. Depending on temperature and conditions, precipitation can arrive as rain, snow, sleet or hail.

Not every cloud produces precipitation that reaches the ground. Falling water can evaporate on the way down, and winds can move clouds far from the place where their moisture first entered the air. The cycle links distant regions, which is why water from one ocean can later fall on another part of a continent. Snow can remain stored on mountains for months before melting, so water may pause in one part of the cycle and move again in a different season.

Once water reaches the ground, gravity takes over. Some flows across the surface as runoff and enters streams, rivers, lakes and oceans. Some soaks into soil, where plants can use it. Water that moves deeper may become groundwater stored in spaces within rock and sediment. Groundwater can move slowly through rock and reappear at springs, feed rivers during dry weather or remain stored underground for years. Human activity can change these paths through paving, pumping, dams and drainage.

The main paths are easy to trace:

• •Evaporation and transpiration add vapour to the air.
• •Condensation forms cloud droplets and ice crystals.
• •Precipitation returns water to the surface.
• •Runoff carries water downhill.
• •Infiltration moves water into soil and rock.

Almost all of Earth's water is salty ocean water. Only a small share is fresh, and most of that fresh water is not in rivers or lakes. It is locked in ice sheets and glaciers or stored underground as groundwater. Rivers and lakes, the sources people often picture first, hold only a tiny fraction of the total. This matters because the water most easily available for drinking and farming is a small, quickly moving part of a much larger system.

Scientists describe each store by its residence time, the average length of time a water molecule stays there. Water in the atmosphere moves quickly, typically remaining only about a week or two before falling as precipitation. Water in rivers may pass through in weeks. Deep groundwater and large ice sheets can hold water for thousands of years. Residence time helps explain why polluted rivers can recover fairly quickly once pollution stops, while contaminated groundwater may stay affected for a very long time.

Changing state takes energy. To turn liquid water into vapour, molecules must absorb a large amount of heat, called latent heat, without getting any warmer. That is why sweating cools you: the evaporating water carries heat away from your skin. When vapour condenses back into droplets, the same energy is released into the surrounding air. So every time water evaporates in one place and condenses somewhere else, heat moves with it.

On a planetary scale this matters a great deal. Huge amounts of water evaporate from warm tropical oceans, and the stored energy is released high in the atmosphere when clouds form, sometimes thousands of kilometres away. That released heat warms the rising air and helps it climb further, which is the engine behind towering thunderstorms. Tropical cyclones are the extreme case. They draw their power from warm sea water evaporating beneath them and weaken quickly over land or cooler water, where that supply of moisture and latent heat is cut off.

Water vapour does not easily condense on its own. A newly formed droplet is so tiny that its molecules escape again quickly, so clean air can hold more vapour than its usual limit without forming cloud. In the real atmosphere, vapour condenses onto microscopic particles called condensation nuclei, such as sea salt, dust, pollen and smoke. Cloud droplets are often only about two hundredths of a millimetre across. A raindrop a couple of millimetres wide holds as much water as roughly a million of them, so a cloud needs some way of gathering its droplets together before rain can fall.

In clouds colder than freezing, ice helps. Many cloud droplets stay liquid well below 0 degrees Celsius, called supercooled water. When ice crystals form among them, the crystals grow quickly, because air that is saturated for liquid droplets is oversaturated for ice, so vapour settles onto the crystals while the droplets slowly evaporate. The heavier crystals fall, collecting more droplets, and often melt into rain on the way down. Much of the rain in mid-latitude countries begins as snow high in the cloud. Cloud seeding tries to exploit this by adding particles such as silver iodide that help ice form.

Rainfall is reported in millimetres, the depth of water that would collect on flat ground if none soaked in, ran off or evaporated. The conversion to volume is pleasingly simple. One millimetre of rain over one square metre is exactly one litre, because a layer 1 millimetre deep over a 1 metre square has a volume of one thousandth of a cubic metre. A roof measuring 10 metres by 5 metres in a downpour of 20 millimetres therefore receives 50 times 20, or 1,000 litres, enough to fill several bathtubs. Over a whole town, the same storm delivers many millions of litres within hours.

Where that water goes depends on the surface. On a meadow or woodland floor, much of it soaks in and reaches streams slowly through soil and groundwater, so rivers rise gently. On roofs, roads and car parks, almost all of it runs off at once into drains. As a result, urban streams rise faster and higher after storms, then drop quickly, with less water left to feed them in dry weather. Engineers now design green roofs, rain gardens, permeable paving and storage ponds to slow the flow, copying the sponge-like behaviour of natural ground. Collecting roof water in butts or tanks works on the same principle.

The water cycle is not a simple circle with one fixed route. It is a network of changing paths powered by sunlight and gravity. Follow the changes of state and the movement downhill, and you can explain how water keeps travelling around the planet.