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.