The deposit of liquid water resulting from condensation of atmospheric water vapor to exposed surfaces that cool during the night. Dewfall is noticeable in the early morning after a calm, cool, clear night, usually as beads of liquid water on the outside and upward-facing surfaces of trees, buildings, and so forth. If the ground is moist, some of the condensed water can be evaporated surface moisture. Dew forms when the surface temperature drops sufficiently to saturate air in contact with the surface (that is, when the surface drops to below atmospheric dew-point temperature); when the surface cools to below freezing temperature, frost occurs. Dew point Frost

Hygroscopic particles on surfaces can act as sites for condensation at temperatures higher than the atmospheric dew-point temperature. Thus, if a surface is not clean, the first deposit of moisture from the air can occur well before the surface cools to dew-point temperature. For some chemicals, such as common salt, condensation can start to occur when the local relative humidity reaches 80%; the humidity must be 100% in the case of a clean surface. Some desert plants exude hygroscopic salts from the interior of leaves which provide preferred sites for condensation and thereby create a supply of water for the plant. Humidity