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Cloud physics
Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric 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 droplets can then collide with small droplets 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.
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