Atmospheric moisture that condenses after a warm day and appears during the night on cool surfaces as small drops. The cool surfaces cause the water vapor in the air to cool to the point where the water vapor condenses.
As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower.
Water condensed onto grass and other objects near the ground, the temperatures of which have fallen below the dewpoint of the surface air due to radiational cooling during the night, but are still above freezing; hoarfrost may form if the dewpoint is below freezing ( see frost point). If the temperature falls below freezing after dew has formed, the frozen dew is known as white dew. The conditions favorable to dew formation are 1) a radiating surface, well insulated from the heat supply of the soil, on which vapor may condense; 2) a clear, still atmosphere with low specific humidity in all but the surface layers, to permit sufficient effective terrestrial radiation to cool the surface; and 3) high relative humidity in the surface air layers, or an adjacent source of moisture such as a lake. Dew plays an important role in the propagation of certain plant pathogens, such as late potato blight, which require dew-covered leaves from certain stages of sporulation. Dew is responsible for the optical effect known as the heiligenschein.
Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that of which it can evaporate, resulting in the formation of water droplets.