| Section D | D index | 201-209 of 573 terms |
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deuterium—Isotope of hydrogen, having one proton and one neutron in the nucleus; heavy hydrogen. Deuterium is a nonradioactive form of hydrogen, occurring naturally with an abundance of about 0.016%. Deuterium is widely used in spectroscopic and kinetic studies, since the large relative mass difference from hydrogen can lead to very different chemical and physical properties of the compounds in which it is contained.
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dew cell—A type of hygrometer used to determine the dewpoint. The equilibrium vapor pressure at the surface of a saturated salt solution is less than that for a similar surface of pure water at the same temperature. This effect is exhibited by all salt solutions but particularly so by lithium chloride. The dew cell hygrometer works in the following manner: The lithium chloride solution is heated until a temperature is reached at which its equilibrium vapor pressure exceeds the ambient vapor pressure. At this point the balance will shift from condensation to evaporation, which can be detected by a characteristic decrease of the conductivity of the solution. By properly regulating the heating current, the equilibrium temperature can be transformed by means of empirical equations to the ambient water vapor pressure and the dewpoint. Provision is not usually made for cooling the dew cell, and consequently it will not measure vapor pressure less than the vapor pressure over saturated lithium chloride solution at the ambient temperature. Dew cells are used for observational purposes, especially for automatic weather stations.
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dew gauge—Instrument for measuring the amount of dew deposited per unit area.
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dew—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.
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dewbow—A rainbow formed in the small drops often found on grass in early morning. While the name implies that those drops are dew, as a practical matter, that is probably rarely the case. Rather, the drops are usually the result of guttation rather than dew. When seen stretched across a meadow, the dewbow may appear in the shape of a hyperbola, but that is merely the intersection of the cone of light that forms the bow and the surface of the meadow. Because they are seen against a surface, dewbows sometimes are perceived as hyperbolas even though, like ordinary rainbows, they are arcs with a constant angular radius.
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