=168. The Formation of Dew.=--If the cooling of the atmosphere is at the surface of some cold object which lowers the temperature of the air below its saturation point, some of its moisture condenses and collects upon the cold surface as _dew_. This may be noticed upon the surface of a pitcher of ice-water in summer. At night, the temperature of grass and other objects near or on the ground may fall much faster than that of the atmosphere owing to the radiation of heat from these objects. If the temperature falls below the saturation point, dew will be formed. This natural radiation is hindered when it is cloudy, therefore little dew forms on cloudy nights. Clear nights help radiation, therefore we have the most dew on nights when the sky is clear. If the temperature is below freezing, _frost_ forms instead of dew. =169. Formation of Fog.=--If the cooling at night is great enough to cool the body of air near the earth below the saturation temperature, then not only may dew be formed, but some moisture is condensed in the air itself, usually upon fine dust particles suspended in it. This constitutes a _fog_. If the cooling of the body of air takes place above the earth's surface as when a warm moist current of air enters a colder region, _e.g._, moves over the top of a cold mountain, or into the upper air, then as this air is cooled below its saturation point, condensation upon fine suspended dust particles takes place, and a _cloud_ is formed. If much moisture is present in the cloud, the drops of water grow in size until they begin to fall and _rain_ results; or if it is cold enough, instead of rain, snowflakes will be formed and fall. Sometimes whirling winds in severe thunderstorms carry the raindrops into colder and then warmer regions, alternately freezing and moistening the drops or bits of ice. It is in this way that _hail_ is said to be formed. =170. The Dew Point.=--The temperature to which air must be cooled to saturate it or the temperature at which condensation begins is called the _dew point_. This is often determined in the laboratory by partly filling a polished metal vessel with water and cooling the water by adding ice until a thin film of moisture is formed upon the outer surface. The temperature of the surface when the moisture first forms is the dew point. =171. The Humidity of the Atmosphere.=--After the dew point has been obtained, one may compute the _relative humidity_ or _degree of saturation of the atmosphere_, from the table given below. This is defined as the _ratio of the amount of water vapor present in the air to the amount that would be present if the air were saturated at the same temperature_.