Dew and frost do not fall from the sky like rain or snow. Rather, they form when air in contact with a relatively cold surface is chilled to saturation. The same process occurs when small drops of water appear on the outside of a cold soft-drink can on a hot summer day. 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.