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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 condensationcondensation,
in physics, change of a substance from the gaseous (vapor) to the liquid state (see states of matter). Condensation is the reverse of vaporization, or change from liquid to gas.
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 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 frostfrost
or hoarfrost,
ice formed by the condensation of atmospheric water vapor on a surface when the temperature of the surface is below 32°F; (0°C;). In the formation of frost, a gas (water vapor) is changed directly to a solid (see dew).
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. 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.



tiny droplets of atmospheric precipitation that form in the evening, at night, and in the early morning at temperatures above freezing on the surfaces of the ground, various objects, and plants. Dew is formed as a result of the cooling of the air and the condensation of water vapor on a given surface; the temperature of the surface falls below the dew point as a result of the heat lost by radiation in the evening and night hours. Heavy dew forms when there is clear weather and a weak wind. The amount of precipitation collected from dew is small—on the average approximately 0.1–0.3 mm nightly.


Water condensed onto grass and other objects near the ground, the temperatures of which have fallen below the dew point of the surface air because of radiational cooling during the night but are still above freezing.


drops of water condensed on a cool surface, esp at night, from vapour in the air
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