Editing Cloud physics (section)

{{short description|Study of the physical processes in atmospheric clouds}}
{{Atmospheric sciences}}
{{Weather}}
'''Cloud physics''' is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds.  These aerosols are found in the [[troposphere]], [[stratosphere]], and [[mesosphere]], which collectively make up the greatest part of the [[homosphere]].  [[Cloud]]s consist of [[microscopic]] droplets of liquid water (warm clouds), tiny crystals of ice (cold clouds), or both (mixed phase clouds), along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei.<ref>{{cite web | url=https://ssec.si.edu/stemvisions-blog/what-are-clouds | title=What Are Clouds? | date=2 March 2017 }}</ref> 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 at a given velocity, the drag force per unit of droplet weight 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 that their downward velocity (relative to the surrounding air) is greater than the upward velocity (relative to the ground) of the surrounding air, the drops can fall as [[precipitation (meteorology)|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 [[hard rime|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 [[weather satellite|satellite]] technology have also allowed the precise study of clouds on a large scale.