Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Clouds are composed of microscopic droplets of water (warm clouds), tiny crystals of ice, or both (mixed phase clouds). Under suitable conditions, the droplets combine to form precipitation, where they may fall to the earth. The precise mechanics of how a cloud forms and grows is not completely understood, but scientists have developed theories explaining the structure of clouds by studying the microphysics of individual droplets. Advances in weather radar and satellite technology have also allowed the precise study of clouds on a large scale.
The amount of water that can exist as vapor in a given volume is proportional to the temperature. When the amount of water vapor is in equilibrium above a flat surface of water the level of vapor pressure is called saturation and the relative humidity is 100%. At this equilibrium there are equal numbers of molecules evaporating from the water as there are condensing back into the water. If the relative humidity becomes greater than 100%, it is called supersaturated. Supersaturation occurs in the absence of condensation nuclei, for example the flat surface of water.
Since the saturation vapor pressure is proportional to temperature, cold air has a lower saturation point than warm air. The difference between these values is the basis for the formation of clouds. When saturated air cools, it can no longer contain the same amount of water vapor. If the conditions are right, the excess water will condense out of the air until the lower saturation point is reached. Another possibility is that the water stays in vapor form, even though it is beyond the saturation point, resulting in supersaturation.
Supersaturation can also occur relative to ice. This is much more common in the atmosphere than supersaturation relative to water. Water droplets are able to maintain supersaturation relative to ice (remain as
ice water droplets and not freeze) because of the high surface tension of each microdroplet, which prevents them from expanding to form larger ice crystals. Without ice nuclei supercooled liquid water droplets can exist down to about -40 C/F, at which point they will spontaneously freeze.
The second critical point in the formation of clouds is their dependence on updrafts. As particles group together to form water droplets, they will quickly be pulled down to earth by the force of gravity. The droplets would quickly dissipate and the cloud will never form. However, if warm air interacts with cold air, an updraft can form. Warm air is less dense than colder air, so the warm air rises. The air travelling upward buffers the falling droplets, and can keep them in the air much longer than they would otherwise stay. In addition, the air cools as it rises, so any moisture in the updraft will then condense into liquid form, adding to the amount of water available for precipitation. Violent updrafts can reach speeds of up to 180 mph (300 km/h). A frozen ice nucleus can pick up 1/2" in size traveling through one of these updrafts and can cycle through several updrafts before finally becoming so heavy that it falls to the ground. Cutting a hailstone in half shows onion-like layers of ice, indicating distinct times when it passed through a layer of super-cooled water. Hailstones have been found with diameters of up to 7" (17.8 cm).
Clouds are classified according to the height at which they are found, and their shape or appearance. The most commonly seen clouds are either "stratiform" (thin, large layer) or "cumuliform" (with vertical development). Some stratus and cumulus clouds are seen at low altitudes of around 2 kilometres. Clouds of similar shape in the topmost region of the troposphere have the prefix "cirro" added to their names ("cirrostratus" and "cirrocumulus"), appearing as light brush strokes in the blue sky, while clouds found at intermediate heights have the prefix "alto" added to their names.
There is also the "cumulonimbus" variety, which is a cloud that virtually spans the entire troposphere from a few hundred metres above the ground up to the tropopause. The cumulonimbus is the cloud responsible for thunderstorms.