The Water Cycle in Everyday Life

The water cycle is usually drawn with oceans, mountains and rain clouds. But every stage of it is a change of state: liquid water turning into gas, gas turning back into liquid or ice, and water moving into the ground. Those changes do not need an ocean. They happen on a washing line, on a cold drink and on a frosty car, following exactly the same rules as in the sky.

One idea links almost everything on this infographic: the air around you always contains some water vapour, even on a clear day. You cannot see it, because water vapour is an invisible gas. What you can see are the droplets and ice crystals that form when that vapour cools.

Evaporation happens when water molecules gain enough energy to break away from the liquid and escape into the air as vapour. Water evaporates fastest at its boiling point, 100 °C, but it evaporates at any temperature, only more slowly when it is cold. That is why a puddle dries up on a mild day and wet clothes dry on a line.

Water evaporates more easily into dry air than into air that already holds a lot of vapour, and wind helps by carrying away the damp air next to a wet surface. So washing dries best on a warm, dry, breezy day. Even on a day below freezing, frozen washing slowly dries as the ice turns straight into vapour. This is called sublimation.

Evaporation also takes heat away. The US National Weather Service explains that when sweat evaporates from your skin, it lowers your body temperature. When the air is very humid, sweat evaporates more slowly, which is one reason a humid day feels hotter than a dry day at the same temperature.

Condensation is the opposite of evaporation. When air containing water vapour is cooled enough, some of the vapour turns back into liquid droplets.

• A cold glass on a humid day gets wet on the outside. The water has not leaked through the glass: it came from vapour in the air that cooled when it touched the cold surface.
• Glasses steam up when you walk from a cool, air-conditioned room into warm, humid air.
• Car windows mist up on the inside, especially after people have been breathing out moist air.
• The white cloud above a boiling kettle is not water vapour. The vapour leaving the kettle is invisible; the white plume is made of tiny liquid droplets that form as it meets the cooler air of the room.

A cloud in the sky forms in a similar way, when rising air cools and vapour condenses onto tiny specks of dust, salt or smoke.

At night, grass, leaves and cars cool down. When a surface cools to the dew point, water vapour in the air touching it condenses into drops of dew. Dew is most likely to form at night, but it can form whenever the dew point is reached.

When the temperature is below freezing (0 °C), something different can happen. Water vapour can change straight into ice crystals on a cold surface, without becoming liquid first. This is deposition, and the feathery white layer it leaves on a windscreen or a lawn is frost. Snowflakes grow by deposition too.

Watch a garden during a shower and you can see infiltration: rain soaking into the soil. How much soaks in depends on the ground. Sandy soil lets water in quickly, while clay absorbs less water and more slowly. Soil that is already soaked from earlier rain behaves like a wet sponge and cannot take much more.

Hard surfaces such as roads and car parks let almost no water in. The rain runs straight off into storm drains, which carry it quickly into streams. That is surface runoff. Water that does soak in can feed groundwater, which seeps slowly into rivers and keeps them flowing even after weeks without rain.

Plants take water up through their roots and release water vapour through tiny pores in their leaves called stomata. This is transpiration, and because the vapour is invisible you normally cannot see it. A clear plastic bag tied over a leafy branch in sunlight makes it visible: the vapour condenses on the inside of the bag, and mist can appear within about an hour.

The amounts are large. The US Geological Survey estimates that a big oak tree can transpire about 151,000 litres of water in a year.