HOPKINS GLOSSARY OF WEATHER TERMS D

deformation-type thermometer

• A thermal sensor (thermometer) with a sensing element that bends or deforms by an amount which is a function of temperature. An example of a deformation-type thermometer is the bimetallic thermometer used in a thermograph.

dewpoint temperature (or dewpoint)

• Temperature to which an air parcel must be cooled at constant pressure and constant water vapor content in order for saturation of that air parcel with water vapor to occur with respect to a liquid water surface. Dewpoint can be measured by a dewpoint hygrometer or indirectly with a psychrometer. The units for dewpoint are the same for air temperature. Compare with frost point.

dew-point hygrometer

• An instrument used for determining the dewpoint; a type of hygrometer operating on the principle that air in contact with a polished metallic surface is cooled to point where a thin film is observed to form on the metal surface, where upon the temperature of that surface is measured. This observed temperature is essentially the same as the theoretical dewpoint .

Doppler radar

• A radar unit that, in the velocity mode, determines the radial movement of airborne hydrometeor or aerosol targets either to or from the unit; based upon the Doppler Effect or Shift, where a slight change in the frequency (or phase) between the broadcast and reflected microwave radiation signaloccurs because the target is moving toward or away from the radar antenna. Precipitation location and intensity are determined also by Doppler radar when operated in a reflectivity mode.

dropsonde

• A modified radiosonde package that is dropped by parachute from an aircraft to obtain temperature, pressure, and humidity profiles of the atmosphere below flight level; often used by aircraft weather reconnaissance of hurricanes.

dry-bulb temperature

• The temperature indicated by the dry-bulb thermometer of a psychrometer; the dry-bulb temperature is identical with the ambient air temperature. Contrast with wet-bulb temperature .

dry-bulb thermometer

• The thermal sensor in a psychrometer that is not moistened, but kept dry. The dry-bulb thermometer indicates or records the dry-bulb temperature. Contrast with wet-bulb thermometer.