The Water Cycle
The water cycle describes the continuous movement of water between the oceans, the atmosphere and the land. Water is not created or destroyed as it moves through the cycle; it changes state and location, driven almost entirely by energy from the Sun and by gravity.
Key terms
- Evaporation
- The change from liquid water to water vapour. It happens fastest where the water is warm, the air is dry and the wind is strong.
- Transpiration
- The release of water vapour from plants, mostly through pores in their leaves called stomata. Together with evaporation it is called evapotranspiration.
- Condensation
- The change from water vapour back to liquid droplets. It occurs when air cools to its dew point, usually because it has risen and expanded.
- Precipitation
- Water falling from cloud to the surface as rain, snow, sleet or hail, once droplets have grown too heavy to stay suspended.
- Infiltration
- The soaking of water into soil and rock. Water that does not infiltrate becomes surface runoff.
The four main stages
- Evaporation. Solar energy warms the surface of oceans, lakes and rivers. Roughly six sevenths of the water entering the atmosphere evaporates from the oceans, the rest from land and vegetation.
- Condensation. Warm, moist air rises and cools. Vapour condenses onto tiny particles of dust, salt or smoke, called condensation nuclei, forming cloud droplets.
- Precipitation. Droplets collide and merge until they are heavy enough to fall. The form they reach the ground in depends on the temperature profile of the air they fall through.
- Collection. Water gathers in oceans, lakes and groundwater. It may be stored for days in the atmosphere or for thousands of years in an ice sheet or a deep aquifer before it moves again.
Where the water is
| Store | Share of all water | Typical residence time |
|---|---|---|
| Oceans | about 96.5% | around 3,000 years |
| Ice caps and glaciers | about 1.7% | up to 100,000 years |
| Groundwater | about 1.7% | weeks to 10,000 years |
| Lakes and rivers | about 0.013% | days to several years |
| Atmosphere | about 0.001% | around 9 days |
Factors that increase evaporation
Four conditions raise the rate at which water leaves a surface as vapour: higher temperature, lower humidity, stronger wind, and a larger exposed surface area. This is why a puddle spread thinly across warm tarmac on a breezy day disappears far faster than the same volume of water standing in a narrow jar.
Why the cycle matters
The cycle moves not only water but energy. Evaporation absorbs latent heat at the surface and condensation releases it high in the atmosphere, which is a major way heat is redistributed from the tropics towards the poles. It also transports sediment and dissolved minerals, shaping river valleys and supplying nutrients to soils and coastal waters.
Check your understanding
- Name the process by which plants return water vapour to the atmosphere.
- Explain why condensation usually happens as air rises.
- A puddle on a warm, windy day evaporates quickly. Give two reasons.
- Water spends about nine days in the atmosphere but thousands of years in the deep ocean. What does this tell you about the size of each store relative to the flow through it?
- Describe one way the water cycle moves energy as well as water.
Further reading
- Introductory physical geography, chapter on atmospheric moisture.
- Any general earth science textbook, section on hydrology.
- Class handout: labelling the stages of the cycle on a diagram.