dew, thin film of water that has condensed on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of overlying air in contact with them, causing the condensation of some water vapor. This condensation occurs because the capacity of air to hold water vapor decreases as the air is cooled. The temperature at which condensation begins, for a sample of air with a given water vapor content, is termed the dew point. If a dew point temperature below 32°F (0°C) is reached, sublimation occurs, i.e., the water vapor converts directly to frost. Should the surface temperature drop below 32°F after the dew has already collected, the dew may freeze into so-called white dew. Most authorities account for the supply of water vapor as coming from the atmosphere, though some research suggests that it also diffuses up through the soil and then condenses on the ground surface if conditions are favorable. Dew forms most readily on those surfaces that lose heat through radiation most efficiently but are nevertheless insulated from external heat sources. Dew formation is favored by high humidity in the lowest layers of air, which either supplies the moisture or at least inhibits the evaporation of the dew already deposited. Strong winds inhibit dew formation because they mix a larger layer of air, creating a more homogeneous distribution of heat and water vapor; under such circumstances it is unlikely that a sufficiently cool and damp layer of air can form near the ground.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.