Condensation and cloud making

Air that rises 1 km cools by 9.8 °C until it is saturated, which is why clouds have flat bases.

A cold glass on a hot day

Take a can of cold drink out of the fridge on a humid day, and within a minute its outside is wet. The water did not leak through the can. It came from the air. Air touching the cold metal cools down, and cool air cannot hold as much vapour as warm air, so the extra vapour condenses into liquid. This is condensation, the opposite of evaporation, and it is exactly how clouds form.

NOAA puts it in terms of two temperatures: the air temperature and the dew point. The dew point is the temperature at which the air becomes saturated. Cool the air below its dew point and vapour starts to condense. When the air temperature and the dew point are equal near the ground, you often get fog.

Rising air cools itself

The most common way to cool air to its dew point is to lift it. Air pressure falls with height, so a rising bubble of air (a parcel) expands. Expanding takes energy from the parcel itself, and it cools even though no heat has left it. Meteorologists call this adiabatic cooling. For dry air the rate is 9.8 °C for every kilometre of rise, the dry adiabatic lapse rate.

For example, surface air is 30 °C with a dew point of 18 °C. As it rises, its temperature falls about 9.8 °C per km and its dew point about 1.8 °C per km. The 12 °C gap closes at 8 °C per km, so the lifting condensation level is roughly 12 ÷ 8 = 1.5 km above the starting height. That is where the first droplets appear, and because all the air in the area has about the same temperature and moisture, it reaches saturation at about the same height. That shared height is the flat base you see under a row of cumulus clouds on a summer afternoon.

Every droplet has a speck at its centre

Cloud droplets usually form when water vapour condenses on aerosol particles called cloud condensation nuclei. Sea salt, dust, smoke and volcanic ash can supply such particles. Activation depends on particle size, composition and how far humidity exceeds saturation, not a universal minimum radius. A growing droplet becomes much larger than its nucleus and is still almost pure water.

Condensation also gives back the energy that evaporation took. When vapour turns to liquid, it releases heat into the surrounding air. That warmth makes the cloudy air more buoyant, so it rises further, condenses more and releases still more heat. NOAA points out that this release helps build hurricanes. A cloud is never finished: at its edges droplets keep evaporating while new ones form inside, which is why clouds change shape as you watch.