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Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Clouds consist of microscopic droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds). Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's Law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.
In warm clouds, larger cloud droplets fall at a higher terminal velocity because the drag force on smaller droplets is larger than on large droplets. The large droplet can then collide with small droplet and combine to form even larger drops. When the drops become large enough so that the acceleration due to gravity is much larger than the acceleration due to drag, the drops can fall to the earth as precipitation. The collision and coalescence is not as important in mixed phase clouds where the Bergeron process dominates. Other important processes that form precipitation are riming, when a supercooled liquid drop collides with a solid snowflake, and aggregation, when two solid snowflakes collide and combine. 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.
History of cloud physics
The history of cloud microphysics developed in the 19th century and is described in several publications. Otto von Guericke originated the idea that clouds were composed of water bubbles. In 1847 Agustus Waller used spider web to examine droplets under the microscope. These observations were confirmed by William Henry Dines in 1880 and Richard Assmann in 1884.
Rising packets of moist air
As water evaporates from an area of the earth surface, the air over that area becomes moist. Moist air is lighter than the surrounding dry air, creating an unstable situation. When enough moist air has accumulated, all the moist air rises as a single packet, without mixing with the surrounding air. As more moist air forms along the surface, the process repeats, resulting in a series of discrete packets of moist air rising to form clouds.
The amount of water that can exist as vapor in a given volume increases with 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 of more than 1–2% relative to water is rarely seen in the atmosphere, since cloud condensation nuclei are usually present. Much high degrees of supersaturation are possible in clean air, and are the basis of the cloud chamber.
Water droplets commonly remain as liquid water and do not freeze, even well below 0 °C (32 °F), because of the high surface tension of each microdroplet, which prevents them from expanding to form larger ice crystals. Without ice nuclei supercooled water droplets can exist down to about −40 °C (−40 °F), at which point they will spontaneously freeze.
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 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 as water droplets have a relatively high surface tension. In addition, the occurrence of collision-coalescence is closely related to entrainment-mixing processes.
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 with respect to ice is lower than the saturation vapor pressure with respect to water. Water vapor interacting with a water droplet may be saturated, at 100% relative humidity, when interacting with a water droplet, but the same amount of water vapor would be supersaturated when interacting with an ice particle. The water vapor 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 between 0 °C (32 °F) and −40 °C (−40 °F). Below −40 °C (−40 °F), liquid water will spontaneously nucleate, and freeze. 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. Cloud seeding adds excess artificial ice nuclei which 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 miles per hour (290 km/h). A frozen ice nucleus can pick up 0.5 inches (1.3 cm) 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 inches (18 cm).
Clouds are classified according to the height at which they are found, and their shape or appearance. There are three basic categories based on physical structure and process of formation. Cirriform clouds are high, thin and wispy, and are seen most extensively along the leading edges of organized weather disturbances. Stratiform clouds appear as extensive sheet-like layers, ranging from thin to moderately thick with some vertical development. They are mostly the product of large scale lift of stable air. Cumuliform clouds are formed mostly into localized heaps, rolls and/or ripples ranging from very small cloudlets of limited convection in slightly unstable air to very large towering free convective buildups when the airmass is very unstable. Clouds of limited convection that show a mix of cumuliform and stratiform characteristics are often grouped into a fourth category, stratocumuliform.
These categories are cross-classified by high, middle, low, and vertical altitude ranges into ten genus types. All cirriform clouds are classified as high and therefore constitute a single cloud genus cirrus. Stratiform and stratocumuliform clouds in the topmost region of the troposphere have the prefix cirro- added to their names yielding the genera cirrostratus and cirrocumulus. Similar clouds found at intermediate heights carry the prefix alto- resulting in the genus names altostratus and altocumulus. No height-related prefixes are used for the low altitudes, so clouds of these two physical categories based around 2 kilometres or lower are known simply as stratus and stratocumulus.
Vertically developed nimbostratus (deep stratiform), cumulus, and cumulonimbus may form anywhere from near surface to intermediates heights of around 3 kilometres and therefore, like the low clouds, have no height related prefixes. However, those capable of producing heavy precipitation or stormy weather carry a nimbo- or -nimbus designation. Of the vertically developed clouds, the cumulonimbus type is the largest and can virtually span the entire troposphere from a few hundred metres above the ground up to the tropopause. It is the cloud responsible for thunderstorms. Its complex structure often combining a cirriform top and stratocumuliform accessory clouds with an overall cumuliform structure sometimes result in this genus type being separated into a fifth physical cumulonimbiform category. This leaves the cumulus genus with its simple heaped structure as the sole purely cumuliform physical category type. Small cumulus is usually considered a low cloud genus, while taller cumulus is more often grouped with cumulonimbiform and deep stratiform genus types as vertical or multilevel.
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