The water cycle
A water molecule that evaporates from the sea stays in the air for about nine days before it falls again. Follow water through the cycle in a 3D block of land: sea, river, mountain and the aquifer under your feet. Change the temperature and watch evaporation and the snow line respond.
About this course
A water molecule that evaporates from the sea stays in the air for about nine days before it falls again. If all the water in the air fell as rain at once, it would cover the globe only about 2.5 cm deep. And there is more fresh water under the ground than in all the world's lakes and rivers together, by more than a hundred times. The cycle looks small from the sky and large from underfoot, which is why this course follows it through a 3D block of land with sea, river, mountain and aquifer.
The six units follow a drop of water around. Where Earth's water is comes first, then evaporation and transpiration: for every 1 °C of warming, air can hold about 7% more water vapour. Condensation and cloud making explains that air rising 1 km cools by 9.8 °C until it is saturated, which is why clouds have flat bases. Rain, snow and the freezing level notes that millions of cloud droplets must join to make one raindrop. Runoff and rivers includes the Great Salt Lake, which is salty because evaporation is the only way water leaves it. The last unit goes down into infiltration and groundwater. In the lab you drag the block to turn it, then move the temperature slider. More dots rise from the sea as it warms. At 5 °C the 0 °C level sits about 0.8 km up, below the cloud base, so snow leaves the cloud and stays frozen. Raise the temperature and the river speeds up.
Before you start
Evaporation, condensation and freezing are changes of state, so States of matter is a good first step. The transpiration section mentions stomata, which Plants explains in more depth. Neither is required, since each idea is introduced again here.
When you finish
You will be able to say where Earth's water is stored and what share sits in the oceans, tell a pool from a flux, and explain why clouds have flat bases. You can describe how droplets grow into raindrops, why the Great Salt Lake is salty, and put the steps of infiltration in order as water moves into the ground and becomes groundwater.
- 1Where Earth's water isIf all the water in the air fell as rain at once, it would cover the globe only about 2.5 cm deep.
- 2Evaporation and transpirationFor every 1 °C of warming, air can hold about 7% more water vapour.
- 3Condensation and cloud makingAir that rises 1 km cools by 9.8 °C until it is saturated, which is why clouds have flat bases.
- 4Rain, snow and the freezing levelMillions of cloud droplets have to join up to make a single raindrop.
- 5Runoff and riversThe Great Salt Lake is salty because evaporation is the only way water leaves it.
- 6Infiltration and groundwaterThere is more fresh water under the ground than in all the world's freshwater lakes and rivers together, by more than a hundred times.