D Damping ratio A constant which describes the performance of a wind vane in response to a step change in wind direction. It is calculated from the relative amount of overshoot on two successive swings (half cycles) of a decaying oscillation. This specification is dimensionless and is generally between 0.3 and 0.7.      Data acquisition The process by which events in the real world are translated into machine-readable signals.      Dead band The range through which the input may be varied without initiating a response. Usually expressed as a percentage of full-scale range.       Decibar A unit of pressure used principally in oceanography. One decibar (105 dynes/cm²) equals 0.1 bar. In the ocean, hydrostatic pressure in decibars very nearly equals the corresponding depth in meters.       Decibel A measure of the relative power, or of the relative values of two flux densities, especially of sound intensities and radar power densities. The decibel is derived from the less frequently used unit, the bel, named in honor of Alexander Graham Bell.     Deformation thermometer A thermometer using transducing elements which deform with temperature. Examples are the bimetallic thermometer and the Bourdon tube type of thermometer.     Degree day Generally, a measure of the departure of the mean daily temperature from a given standard; one degree day for each degree (°C or °F) of departure above (or below) the standard during one day. Degree days are accumulated over a "season." As used by the U.S. Army Corps of Engineers, degree days are computed above and below 32°F, positive if above and negative if below.       Degree hour As used by the U.S. Army Corps of Engineers, the departure (in °F) of the hourly temperature form a standard 32°F, positive if above and negative if below. Degree hours may be accumulated over any period of time, depending upon the use to which they are applied.      Deepening A decrease in the central pressure of a pressure system. Usually applied to a low rather than to a high.    Degree-day A unit that represents one degree of deviation from a reference point in the mean daily outdoor temperature (usually 65°F) and that is used to measure heating and cooling requirements.     Delay distance The length of air flow past a wind vane required for the vane to respond to 50 percent of a step change in wind direction. Expressed in feet or meters and calculated from delay time times wind tunnel speed.     Delta temperature The difference between temperature measurements taken at two significant levels above the ground. Temperatures at 10 and 40 meters are commonly used.     Depression In meteorology, an area of low pressure; a low or trough.     Dew Water condensed onto objects at or near the ground, due to the fact that their temperatures have fallen below the dew point temperature of the surrounding air, but not below freezing.    Dew cell An instrument used to determine dew point.    Dew point (or dew-point temperature) The temperature to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation by water vapor. When this temperature is below 0°C, it is sometimes called the frost point.     Dew-point apparatus Same as dew-point hygrometer.     Dew-point hygrometer Hygrometer in which the dew (frost) point is determined by observing the temperature of an artificially cooled surface at the moment at which dew (frost) first appears on it.    Dew-point spread The difference between the air temperature and the dew-point. Also called dew-point deficit, dew-point depression.     Diabatic process Thermodynamic change of state of a system in which there is transfer of heat across the boundaries of the system. Compare to adiabatic process.     Diamond-Hinnman radiosonde A variable audio-modulated radiosonde developed at the Bureau of Standards and used by the United States weather services.     Diffuse solar radiation (sky radiation) Downward scattered and reflected solar radiation, coming from the whole hemisphere with the exception of the solid angle of the sun's disc on a surface perpendicular to the axis of this cone.     Diffusion hygrometer A hygrometer based upon the diffusion of water vapor through a porous membrane.     Digital Pertaining to measurements or devices in which the output varies in discrete steps, i.e. on-off or pulse signals. Compare to analog.     Dines anemometer A type of pressure-tube anemometer, named after the inventor.     Dines radiometer An instrument for measuring radiant energy. It consists of an ether differential thermometer with blackened bulbs. One of the bulbs is exposed to the unknown radiation and the other to a black body source whose temperature can be varied. Equality of radiation is indicated by the balance of the differential thermometer.    Direct solar radiation Radiation coming from the solid angle of the sun's disc, as opposed to diffuse sky radiation, effective terrestrial radiation, or radiation from any other source. Direct solar radiation is measured by pyrheliometers.     Discharge Rate of flow of water past a point in a stream, expressed as volume per unit time, i.e. cubic feet per second.     Disdrometer Apparatus designed to measure and record the size distribution of raindrops as they occur in the atmosphere.     Disk hardness gauge An instrument for measuring snow hardness in terms of the resistance of snow to the pressure exerted by a disk attached to a spring-loaded rod, a gauge calibrated in pounds per square inch registers the amount of resistance. See Canadian hardness gauge.     Distance constant The length of fluid flow (gas or liquid) past a sensor required for the sensor to respond to 63.2% of a step change in speed. Expressed in feet or meters. For anemometers, this value is calculated from time constant times wind tunnel speed.     Dobson spectrophotometer A photoelectric spectrophotometer which is used in the determination of the ozone content of the atmosphere.     Doppler radar Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna). Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was moving away.     Dosimeter An instrument for measuring the ultraviolet in solar and sky radiation.     Double-theodolite observation A technique for making winds aloft observations in which two theodolites located at either end of a baseline follow the ascent of a pilot balloon. Synchronous measurements of the elevation and azimuth angles of the balloon, taken at periodic intervals, permit computation of the wind vector as a function of height.     Downdraft A relatively small-scale, downward moving current of air.     Downward total radiation Solar and terrestrial radiation directed downwards (towards the earth's surface); incoming radiation.     Downwind The direction toward which the wind is blowing; with the wind.     Drainage area The size of the area comprising a watershed or river basin. Also called catchment area. Drizzle Very small precipitation drops (diameters less than 0.5 mm) that appear to float with air currents while falling in an irregular path. Unlike fog droplets, drizzle falls to the ground.    Dropsonde A radiosonde which is dropped by parachute from an aircraft for the purpose of obtaining soundings of the atmosphere below.     Drosometer An instrument used to measure the amount of dew formed on a given surface.    Dry adiabatic lapse rate The rate of decrease of temperature with height when unsaturated air is lifted adiabatically (without exchange of heat with its surroundings). The decrease is due to expansion as the air is lifted to a lower pressure.     Dry-bulb temperature Technically, the temperature registered by the dry-bulb thermometer of a psychrometer. However, it is identical with the temperature of the air and may also be used in that sense.     Dry-bulb thermometer Companion to the wet-bulb thermometer in a psychrometer. Used to measure ambient air temperature.      Dyne The unit of force in the centimeter-gram-second system of physical units, i.e. one gm cm per sec2, equal to 7.233 x 10-5 poundal.