How Clouds Form: From Water Vapour to Droplets and Ice

Air can rise for several reasons. The Sun may warm the ground so nearby air becomes buoyant, mountains can force air upward, or weather systems can lift one air mass over another. As rising air enters regions of lower pressure, it expands. Expansion uses energy and causes the air to cool.

Cooler air can reach a point where it becomes saturated with water vapour. The temperature at which this happens is related to the dew point. If cooling continues, some water vapour changes into liquid droplets or, at sufficiently low temperatures, ice crystals.

This cooling process is central to cloud formation. Simply having water vapour is not enough. Air must usually reach conditions that allow condensation or deposition to produce many tiny visible particles.

Water molecules do not usually form cloud droplets efficiently in perfectly clean air. They gather around microscopic particles called cloud condensation nuclei. Dust, sea salt, smoke, and other aerosols can provide surfaces where water begins to collect.

Each droplet is extremely small, so air currents can keep it suspended. A cloud may contain huge numbers of droplets while still feeling light and misty. The cloud becomes visible because these droplets and ice crystals scatter incoming light.

Cloud shape depends on the movement and stability of the surrounding air. Strong rising currents can build tall clouds, while gentle lifting across a broad layer may create flatter sheets. Understanding cloud formation therefore requires both the water cycle and the behaviour of the atmosphere.

A cloud does not automatically produce rain. Several steps may follow:

• •Cloud droplets collide and combine into larger drops.
• •Ice crystals can grow by collecting water vapour or smaller droplets.
• •Larger particles begin to fall when gravity overcomes upward air motion.
• •Falling precipitation can evaporate before reaching the ground.
• •Temperature determines whether precipitation arrives as rain, snow, or another form.

Clouds can form at very different heights and temperatures, which is why they do not all look alike. Puffy cumulus clouds often grow where local convection lifts warm air, while layered stratus clouds can form when broad areas of air rise gently or cool near the surface. High clouds may consist largely of ice crystals. These names describe visible forms, but the same basic physics sits underneath them. Start with rising or cooling air and then use atmospheric conditions to explain the cloud type you see.

Humidity matters too. Air that already contains a large amount of water vapour needs less cooling to reach saturation than much drier air, so the starting moisture helps determine when cloud formation begins.

Fog is simply cloud at ground level, but it usually forms without air rising. On a clear, still night the ground loses heat by radiating it into space, and the air just above it cools until it reaches its dew point. Droplets then form close to the surface, which is why radiation fog often fills valleys and hollows at dawn and clears as the Sun warms the ground. Sea fog forms differently, when moist air drifts over cold water.

Aircraft contrails are clouds made partly by the engines themselves. Burning fuel produces water vapour and tiny particles, and in the very cold air at cruising height the vapour condenses and freezes almost immediately. Whether a trail vanishes quickly or spreads out depends on how humid the upper air already is. You can see the basic physics at home by squeezing a bottle containing a little warm water and a trace of smoke, then releasing it quickly so the air expands and cools.

Cloud droplets do not freeze the moment the temperature drops below zero. Tiny droplets can stay liquid, or supercooled, far below freezing, sometimes to around minus 35 to minus 40 degrees Celsius, because ice needs a suitable particle to start forming on, and such ice nuclei are much rarer than the particles water condenses on. In a cloud that mixes supercooled droplets with a few ice crystals, the crystals grow while the droplets shrink, because air that is just saturated for liquid water is oversaturated for ice. This is one of the main ways rain and snow begin in cold clouds.

In 1946 Vincent Schaefer, working at the General Electric laboratory in New York, dropped dry ice into a chilled box containing a supercooled cloud made from his breath and watched it fill with sparkling ice crystals. His colleague Bernard Vonnegut soon found that silver iodide, whose crystal structure resembles ice, could start freezing too. These discoveries launched cloud seeding, which several countries still use to try to increase rain or snow or reduce hail. Proving how much it helps is difficult, because natural rainfall varies so much that a modest effect is hard to separate from chance.

A fair-weather cumulus cloud looks weightless, but a rough calculation shows otherwise. A typical value for the liquid water in such a cloud is about half a gram per cubic metre. A modest cloud measuring one kilometre on each side contains a billion cubic metres of air, so it holds about 500 million grams of water, or around 500 tonnes. It stays up because that water is spread between countless droplets, each so small that it falls through still air at around a centimetre per second or less, a speed easily matched by the gentle rising air that built the cloud.

Those droplets are also the reason clouds look white. Air molecules are far smaller than the wavelength of light and scatter blue light much more strongly than red, which is why a clear sky is blue. Cloud droplets are many times larger than the wavelength of light, so they scatter all colours roughly equally, and the mixture looks white. A thick cloud scatters so much sunlight back out through its top and sides that relatively little reaches its base. Seen from below, the underside therefore appears grey, and the deeper the cloud, the darker it looks, which is why tall storm clouds can seem almost black.

On a sunny afternoon, heaped cumulus clouds often share a flat, level base, as though sliced with a ruler. That base marks the height where rising air has cooled to its dew point, called the lifting condensation level, and you can estimate it with simple arithmetic. Unsaturated rising air cools at roughly 10 degrees Celsius for every kilometre it climbs, while its dew point falls by only about 2 degrees per kilometre. The gap between temperature and dew point therefore closes by around 8 degrees per kilometre, or 1 degree for every 125 metres.

Suppose the surface temperature is 24 degrees Celsius and the dew point is 16 degrees. The spread is 8 degrees, so the cloud base should sit about 1,000 metres up. On a humid day with a spread of only 2 degrees, cloud forms at around 250 metres, which is why muggy days often bring low, ragged cloud. Above the base, things change. Condensation releases the heat the water absorbed when it first evaporated, so saturated air cools more slowly as it rises, often by roughly 5 to 6 degrees per kilometre. That released heat keeps the air buoyant, which is how cumulus clouds can tower upwards.

How clouds form can be followed in a simple chain: moist air rises, expanding air cools, water vapour reaches saturation, and droplets or ice crystals develop around tiny particles. Weather conditions then shape the cloud and determine whether its particles grow large enough to fall. Follow the air's movement and temperature, and cloud formation becomes much easier to picture.