Most clouds occur within the troposphere. Because air temperature drops with increasing altitude within the troposphere, high clouds are extremely cold and consist mostly of ice crystals. The crystals give these clouds a fuzzy appearance. Low clouds are warmer and generally are composed of droplets. These clouds appear to have sharper edges.
Clouds usually form where air moves upward.
As air ascends, it encounters lower and lower pressure. Air responds to lower pressure by expanding. Whenever gases expand, they cool. As air cools, its relative humidity increases until it reaches saturation and clouds form. Where air moves downward, clouds usually do not develop.