1. Quick Summary
A cloud is not a gas. It is a swarm of tiny liquid water droplets or ice crystals, small enough that air currents keep them aloft and large enough that they scatter light and look white.
Clouds form when air rises, expands because pressure drops with altitude, and therefore cools. Once the air reaches its dew point, water vapour condenses onto microscopic particles called cloud condensation nuclei.
2. What It Means
The key idea is that warm air can hold more water vapour than cold air. Cooling air does not ‘make room smaller’ for water — it lowers the threshold at which vapour turns back into liquid.
That threshold is the dew point. When the temperature of a rising parcel of air falls to its dew point, relative humidity reaches 100% and condensation begins.
Crucially, water vapour almost never condenses into droplets on its own in perfectly clean air. It needs a surface. Dust, sea salt, smoke and pollen provide that surface.
3. Why It Happens
Air rises for four main reasons: the sun heats the ground and warm air convects upward; air is forced up a mountain slope; a warm air mass slides over a cold one along a front; or air converges and has nowhere to go but up.
As a parcel rises into lower pressure it expands. Expansion uses energy, and that energy comes from the parcel’s own heat, so the parcel cools. This is called adiabatic cooling — roughly 10 °C per kilometre for dry air.
Cloud condensation nuclei are typically 0.0001 to 0.01 millimetres across. Without them, air would have to reach several hundred percent humidity before droplets could form spontaneously.
The droplets in a cloud are about 0.01 to 0.02 millimetres in diameter. They fall at roughly one centimetre per second, while even gentle updrafts inside a cloud move at tens of centimetres per second — so the droplets are constantly being carried up faster than they fall.
4. Real Examples
Cumulus clouds, the puffy fair-weather kind, mark individual rising thermals. Their flat bottoms show the exact altitude where rising air hits its dew point.
Stratus clouds form when a broad layer of air is lifted gently or cooled from below, producing a flat grey sheet rather than separate towers.
Lenticular clouds form downwind of mountains when air is forced up and over a ridge, then sinks again — they look like stacked lenses and often stay in one place.
Contrails are artificial clouds: hot jet exhaust adds both water vapour and soot particles to cold high-altitude air, and the vapour immediately condenses and freezes.
5. How It Affects Us
Clouds control the planet’s temperature balance. Low clouds mostly reflect sunlight and cool the surface; high thin clouds let sunlight through but trap infrared radiation and warm it.
Clouds are the delivery step of the water cycle. Every drop of rain starts as a cloud droplet that grew large enough — about 0.5 millimetres — to fall faster than the updraft holding it.
Weather forecasting depends on predicting cloud formation, which means predicting vertical air motion and humidity — the two hardest things to measure well.
6. Key Takeaways
- Clouds are liquid or solid water, not vapour. The vapour form is invisible.
- Rising air cools by expansion, and cooling air eventually reaches its dew point.
- Condensation needs microscopic particles to start on; clean air can stay supersaturated without forming a cloud.
- Cloud droplets stay up because updrafts outpace their very slow fall speed, not because they are weightless.