Cloud physics

Cloud physics

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.

Formation

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

Supersaturation of more than 1-2% relative to water is rarely seen in the atmosphere. For high levels of supersaturation there must be no condensation nuclei for the water vapor to condense on.

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.

Collision-Coalescence

One theory explaining how the behavior of individual droplets leads to the formation of clouds is the collision-coalescence process. Droplets suspended in the air will interact with each other, either by colliding and bouncing off each other or by coalescing -- combining -- to form a larger droplet. Eventually, the droplets become large enough that they fall to the earth as precipitation. The collision-coalescence process does not make up a significant part of cloud formation for the same reason that water droplets have a relatively high surface tension, which prevents them from coalescing on a large scale before they eventually fall to the earth.

Bergeron Process

The primary mechanism for the formation of ice clouds was discovered by Tor Bergeron. The Bergeron process notes that the saturation vapor pressure of water, or how much water vapor a given volume can hold, depends on what the vapor is interacting with. Specifically, the saturation vapor pressure of air with respect to ice is lower than the saturation vapor pressure with respect to water. Air interacting with a water droplet may be saturated (at 100% RH) when interacting with a water droplet, but the same air would be supersaturated when interacting with an ice particle. The air will attempt to return to equilibrium, so the extra water vapor will condense into ice on the surface of the particle. These ice particles end up as the nuclei of larger ice crystals. This process only happens at temperatures around -40 °C. The surface tension of the water allows the droplet to stay liquid well below its normal freezing point. When this happens, it is now supercooled liquid water. The Bergeron process relies on supercooled liquid water interacting with ice nuclei to form larger particles. If there are few ice nuclei compared to the amount of SLW, droplets will be unable to form. A process whereby scientists seed a cloud with artificial ice nuclei to encourage precipitation is known as cloud seeding. This can help cause precipitation in clouds that otherwise may not rain. Adding excess artificial ice nuclei -- overseeding a cloud -- shifts the balance so that there are many nuclei compared to the amount of supercooled liquid water. An overseeded cloud will form many particles, but each will be very small. This can be done as a preventative measure for areas that are at risk for hail storms.

Dynamic Phase Hypothesis

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).

Classification of Clouds

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.

References

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