Köhler theory describes the process in which water vapor condenses and forms liquid cloud drops, and is based on equilibrium thermodynamics. It combines the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface, and Raoult's Law, which relates the saturation vapor pressure to the solute. It is an important process in the field of cloud physics. It was initially published in 1936 by Hilding Köhler, Professor of Meteorology in the Uppsala University.
Köhler equation:
where  is the droplet water vapor pressure,  is the corresponding saturation vapor pressure over a flat surface,  is the droplet surface tension,  is the density of pure water,  is the moles of solute,  is the molecular weight of water, and  is the cloud drop diameter.