How Do Clouds Form?

The sun heats the ground.
The warm ground in turn heats the air above. (In the above diagram, the warm ground and air are shaded in red.)
The warm air at the ground is lighter and less dense than the surrounding air above, so it begins to rise.
The rising air takes on the shape of a round "parcel."
Several things happen as the parcel rises up through the atmosphere:
- The higher up into the atmosphere you go, the lower the pressure. As the parcel of air rises, it experiences this drop in pressure. As the pressure on the air parcel drops, the parcel expands and increases in size. (In the diagram I drew the rising parcel getting bigger as it rises.)
- Anytime air expands due to a drop in pressure, it also gets cooler. So, as the parcel rises and expands, it also cools. (In the diagram, the green and blue colors indicate the parcel getting cooler.) The parcel cools at a rate of approximately 10 degrees C per 1000 meters of rise (or 5.5 degrees F per 1000 feet of rise.) This is called the dry adiabatic lapse rate.
- Warm air is able to hold larger amounts of water vapor (water in gas form) than cool air. The cooling parcel is no longer able to hold all of the water vapor it was able to hold when it was warm. The excess water vapor begins to condense out of the air parcel in the form of liquid cloud droplets. When water vapor begins to condense into liquid droplets (moisture), this means the air parcel is saturated and has reached its dewpoint temperature. The higher the parcel rises, the more it expands, the cooler it gets, and the greater the amount of moisture that condenses out of it.
The rising, expanding, cooling air parcel finally rises so high that the surrounding atmospheric air is the same temperature as the cooled parcel. This is called equilibrium temperature. When equilibrium is reached, the parcel loses momentum and stops rising.
By the time the air parcel has reached equilibrium temperature, enough moisture has condensed out of it to form a visible cloud!
Cloud heights and shapes vary according to the atmospheric conditions. On a hot day, clouds will tend to form higher, because the rising air parcels must rise higher in order to cool enough to reach dewpoint. When air parcels continue to rise after dewpoint has been reached and condensation has occurred, the cloud becomes very tall. This is how cumulus congestus and cumulonimbus clouds form. If air parcels only rise a short distance over a large geographical area after reaching dewpoint, layer clouds (such as stratus) will form.