Evaporation

Evaporation is the process by which liquid water turns into water vapour, an invisible gas, and rises into the air. It is the first step of the water cycle and one of the most important things happening on the planet. Every day the Sun evaporates approximately 1.4 trillion tonnes of water from the oceans, lakes, rivers and damp ground around the world. That water goes up into the air, eventually forms clouds, and falls back down somewhere else as rain or snow.

  • Water evaporated dailyApprox. 1.4 trillion tonnesFrom oceans, lakes, rivers, plants
  • Energy requiredApprox. 2,260 kJ per kgTo turn water from liquid to gas at 100 °C
  • Powered byThe SunApprox. 50% of solar energy goes into evaporation
  • From oceansApprox. 86%Of all evaporation worldwide
  • From plantsApprox. 10%Called transpiration
  • Speeds it upHeat, wind, dry air, surface area

How evaporation works

Water molecules in a liquid are constantly moving, bumping into each other. A few of them, near the surface, have enough energy to break free of the others and escape into the air as water vapour. The hotter the water, the faster the molecules move, and the more of them can escape. That is why water evaporates faster on a hot day than a cold one. Importantly, evaporation can happen at any temperature, not just at 100 °C (the boiling point). Boiling is just evaporation that has become so fast it happens throughout the liquid, not just at the surface.

What speeds evaporation up

  • Heat: warmer water evaporates faster.
  • Wind: blows water vapour away from the surface, allowing more molecules to escape. This is why laundry dries faster on a windy day.
  • Dry air: air that is not already saturated with water can absorb more. This is why washing dries faster in dry climates.
  • Surface area: more surface exposed means more molecules can escape. Spread your washing out, do not bunch it up.
  • Low air pressure: water boils at lower temperatures at high altitude.

How sweating cools you down

The water molecules that escape during evaporation carry away energy with them, leaving the remaining liquid slightly cooler. This is the principle behind sweating. When you get hot (from exercise or hot weather), your body releases water onto your skin. As that water evaporates into the air, it carries heat away and your skin cools down. This is also why a wet T-shirt feels cold, why fans work by speeding up evaporation off your skin, and why you feel cold getting out of a swimming pool on a windy day.

Special types of evaporation

  • Transpiration: when plants release water vapour through tiny pores in their leaves. A mature oak tree can release approx. 150 litres of water per day this way.
  • Sublimation: a special case where solid ice turns directly to water vapour without ever becoming liquid. Happens slowly to snow and frost on cold dry days.
  • Evapotranspiration: the combined evaporation from land surfaces plus transpiration from plants. The total amount of water rising into the air from a forest, for example.
Fact The deserts of the Middle East and North Africa lose so much water to evaporation that some lakes there have negative net rainfall: even when it does rain a little, more water evaporates away than the rain brings in. This is why salt lakes (like the Dead Sea) keep getting saltier: only the water leaves; the dissolved salt is left behind, building up over thousands of years.

Why evaporation matters for the planet

Evaporation is far more than just one stage of the water cycle. It plays a huge role in shaping Earth's climate and weather.

  • Cooling: evaporation from oceans, rivers and plants cools the surface of the planet.
  • Heat transport: water vapour carries heat upwards and (later) across continents.
  • Cloud formation: without evaporation there would be no clouds and no rain.
  • Atmospheric circulation: differences in evaporation rates between tropics and poles drive much of the wind patterns.
Did you know? Around 50% of all the solar energy Earth receives is used to evaporate water. That sounds like a lot, but it makes sense when you think about how much water moves through the atmosphere every day. All that energy is later released when the water vapour condenses back into clouds, which is partly why storms can be so powerful.
Deeper dive: why hurricanes are evaporation in extreme form

A hurricane (or typhoon, or tropical cyclone) is essentially a giant heat engine powered by evaporation. They form over warm tropical oceans, where the sea surface temperature is above approximately 26.5 °C.

Here is how it works. Warm sea water evaporates rapidly into the air, carrying enormous amounts of heat energy in the water vapour. As the warm wet air rises and cools, the water vapour condenses back into liquid (forming clouds and rain) and releases its stored heat. This released heat warms the surrounding air, which rises further, sucking in more warm wet air from below. The whole system spirals upwards and outwards, forming the giant rotating storm we call a hurricane.

The energy involved is huge. A typical hurricane releases as much heat energy in a single day as 200 times the world's electricity consumption. Almost all of that energy started as warm water vapour evaporating off the tropical ocean. This is also why hurricanes weaken so quickly when they move over land or cold water: they lose their fuel supply.

This is also one of the worrying things about climate change. Warmer ocean surfaces mean more evaporation, more water vapour in the air, and more energy available to storms. Most climate scientists expect hurricanes to get stronger (though not necessarily more frequent) as the world warms.

For the next stage, see condensation. For the overview, see what is the water cycle.