It can be said that hydroscopic nuclei have an affinity for water or that they readily absorb and retain water. Some dust par-ticles are also hydroscopic, but not effectively so. Air has been supersaturated in laboratories to over 400 percent before conden-sation began when there were no hydroscopic nuclei present. On the other hand, condensation has been induced with relative humidities of only 70 percent when there was an abundance of hydroscopic nuclei. The condensation which results when all three mentioned conditions are fulfilled is usually in the form of mist, clouds, or fog.
Fogs are merely clouds on the surface of Earth.
In our industrial cities, where byproducts of combustion are abundant, the distinction between smoke, fog, and haze is not easily discernible. A combination of smoke and fog gives rise to the existence of the so-called smogs characteristic of these industrial areas. Little is known about the properties of sublimation nuclei, although it is believed they are essential for sublimation to occur at all. It is assumed sublimation nuclei are very small and very rare, possibly of a quartz or meteoric dust origin. All cirriform clouds are composed of ice crystals and are believed to be formed as a result of direct sublimation.