Cloud Physics

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of clouds. Cloud formations are composed of microscopic droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds). Cloud drops initially grow by the condensation of water vapor onto the drop when the supersaturation of an air parcel exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud drop formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the 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 drop is 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.