Condensation

Condensation is the opposite of evaporation: it is when water vapour in the air cools down and turns back into tiny liquid water droplets. It is the second stage of the water cycle and is responsible for the formation of clouds, dew, fog and the mist on your bathroom mirror after a hot shower. Without condensation, the water that rises from the oceans would never come back down and the Earth would soon run out of fresh water on land.

  • ProcessWater vapour to liquidTriggered by cooling
  • Where it happensAnywhere warm wet air meets coldAir, glass, ground, leaves
  • Cloud dropletsApprox. 0.02 mm wideNeed millions to make a raindrop
  • Dew pointTemperature at which it startsDepends on how much water is in the air
  • Releases heatApprox. 2,260 kJ per kgSame as evaporation takes in
  • Powers weatherA huge amountMost storms are powered by released heat

How condensation works

Warm air can hold more water vapour than cold air. When warm wet air cools down (by rising into colder altitudes, or meeting a cold surface), it can no longer hold all its water vapour. The extra water vapour condenses: the molecules slow down enough to stick together as tiny liquid droplets.

The temperature at which condensation starts is called the dew point. It depends on how much water is in the air. Very humid air can reach its dew point with only a small cooling; very dry air needs to be cooled a lot more. When the air temperature reaches the dew point, condensation starts forming on any cool surface around.

How clouds form

Almost all clouds form by condensation. Here is the basic story.

  1. The Sun warms the ground (or the sea) and the air just above it.
  2. That warm wet air rises, because warm air is less dense than cold air around it.
  3. As the air rises higher, it expands and cools. (Rising air always cools as it expands.)
  4. Eventually it cools below the dew point, and water vapour starts condensing into tiny droplets.
  5. Billions of droplets together make a visible cloud.

Cloud droplets are extremely small (approx. 0.02 mm wide, about the width of a fine human hair). It takes roughly a million of them combining together to make a single raindrop.

Everyday condensation

You see condensation all the time.

  • Bathroom mirror after a hot shower: warm wet air meets the cool mirror, water condenses on the surface.
  • Cold glass of water on a hot day: water vapour in the warm room condenses onto the cold glass.
  • Breath on a cold morning: the warm wet air from your lungs hits cold air and the water vapour condenses into a visible cloud.
  • Dew on grass in the morning: warm air loses heat overnight, cooling below the dew point and condensing on the cold grass.
  • Fog: when condensation happens at ground level rather than higher up in the sky.
  • Aircraft contrails: hot wet jet exhaust meeting cold high-altitude air.
Fact Condensation releases heat. As water vapour turns back into liquid, it gives up the same amount of heat (approx. 2,260 kJ per kg) that it absorbed during evaporation. This released heat is what powers most of the world's major storms. Hurricanes, thunderstorms and tornadoes all draw their energy from water vapour condensing high in the atmosphere.

What are condensation nuclei?

In pure, perfectly clean air, water vapour can be cooled well below its dew point without condensing. To condense efficiently, the water molecules need a tiny solid particle to stick to. These are called condensation nuclei and can include dust, sea salt, smoke, pollen, microscopic ash and pollution particles.

This is why some volcanic eruptions and forest fires lead to more rain downwind: extra particles in the air give water vapour more places to condense.

Did you know? Cloud seeding is a technique where small particles (often silver iodide) are sprayed into clouds to provide extra condensation nuclei and encourage rain. Several countries (including China, the UAE and the United States) use cloud seeding to try to increase rainfall in dry areas. The technique works in some conditions but not others, and how much extra rain it really produces is still debated.
Deeper dive: cloud types and what they tell you about the weather

Different conditions in the atmosphere create different kinds of clouds, and learning to read them is one of the oldest forms of weather forecasting. The basic cloud classification was set up by English chemist Luke Howard in 1802 and is still used today. There are 10 main cloud types, named in Latin.

The basic family names tell you a lot.

  • Cirrus ("curl of hair"): thin wispy clouds high in the sky. Made of ice crystals. Often a sign of approaching weather changes.
  • Cumulus ("heap"): the puffy cotton-wool clouds of a fine day. Form when warm air rises in columns and condenses at a particular height.
  • Stratus ("layer"): smooth flat sheets of cloud, often grey and gloomy. Form when wet air cools slowly over a wide area.
  • Nimbus ("rain"): the prefix or suffix for any cloud producing rain. Nimbostratus = layered rain cloud; cumulonimbus = heaped storm cloud.

Combinations of these give you the 10 main types: cirrus, cirrocumulus, cirrostratus, altocumulus, altostratus, nimbostratus, stratocumulus, stratus, cumulus, cumulonimbus. The towering anvil-shaped cumulonimbus is the cloud responsible for thunderstorms; learning to recognise it is genuinely useful if you spend time outdoors.

For the next stage, see precipitation. For where condensation comes from, see evaporation.