- 行业: Weather
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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
An instrument for determining the dewpoint. The most widely used systems employ a small polished-metal reflecting surface, cooled and heated electrically using a Peltier-effect device, and a small temperature sensor embedded on the underside of the mirror. An electro-optical system is used to detect the formation of condensate and to provide the input to the servo-control system to regulate the temperature of the mirror. The specular reflectance of the bare mirror decreases with dew layer thickness; a preset threshold defines a specific constant dew layer thickness that is maintained by the optical servo-control. The mirror temperature equals the dewpoint.
Industry:Weather
A formula for the calculation of the approximate height of the lifting condensation level. Therefore, it is employed to estimate the height of the base of convective clouds, under suitable atmospheric and topographic conditions. In a practical but simplified form, it may be stated
where H is the height above the surface in km and T and Td are the temperature and dewpoint in degrees Celsius. This is derived from assuming that the temperature decreases 9. 8°C and the dewpoint decreases 1. 8°C for each km increase in altitude, values which are approximately correct for a well-mixed boundary layer. The convective-cloud height diagram is a graphic form of this relationship.
Industry:Weather
The difference in degrees between the air temperature and the dewpoint.
Industry:Weather
In dry deposition, the quotient of the flux of a particular species to the surface (in units of concentration per unit area per unit time) and the concentration of the species at a specified reference height, typically 1 m. Typical deposition velocities for common gas phase pollutants (e.g., ozone, nitric acid) are of order 0. 01–5 cm s−1.
Industry:Weather
The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur. When this temperature is below 0°C, it is sometimes called the frost point. The dewpoint may alternatively be defined as the temperature at which the saturation vapor pressure of the parcel is equal to the actual vapor pressure of the contained water vapor. Isobaric heating or cooling of an air parcel does not alter the value of that parcel's dewpoint, as long as no vapor is added or removed. Therefore, the dewpoint is a conservative property of air with respect to such processes. However, the dewpoint is nonconservative with respect to vertical adiabatic motions of air in the atmosphere. The dewpoint of ascending moist air decreases at a rate only about one-fifth as great as the dry-adiabatic lapse rate. The dewpoint can be measured directly by several kinds of dewpoint hygrometers or it can be deduced indirectly from psychrometers or devices that measure the water vapor density or mixing ratio. See dewpoint formula.
Industry:Weather
A climate type that is characterized by insufficient moisture to support appreciable plant life - that is, a climate of extreme aridity.
Industry:Weather
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.
Industry:Weather
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.
Industry:Weather
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.
Industry:Weather
