Clouds are a direct expression of the physical processes taking place in the atmosphere. An accurate description of both type and amount plays an important part in the analysis of the weather and in forecasting the changes that take place in the weather. To be able to thoroughly understand clouds, the Aerographer s Mate must know the physical processes that form clouds. Three conditions must be met before clouds can form as a result of condensation presence of sufficient moisture, hydroscopic or sublimation nuclei in the atmosphere, and a cool-ing process. The first task is to consider the hydroscopic and sublimation nuclei.
Hydroscopic nuclei are particles of any nature on which condensation of atmospheric moisture occurs.
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.