The temperature to which air must be cooled to reach saturation and produce dew is known as the dew point . A cloud that is in contact with the earth's surface is known as fog. 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.