Aeronautical Terms beginning with D

Density altitude

The altitude in the standard atmosphere at which the air has the same density as the air at the point in question. An aircraft will have the same performance characteristics as it would have in a standard atmosphere at this altitude.


Deepening

A decrease in the central pressure of a pressure system; usually applied to a low rather than to a high, although technically, it is acceptable in either sense.


Density

(l) The ratio of the mass of any substance to the volume it occupies—weight per unit volume. (2) The ratio of any quantity to the volume or area it occupies, i.e., population per unit area, power density.


Depression

In meteorology, an area of low pressure; a low or trough. This is usually applied to a certain stage in the development of a tropical cyclone, to migratory lows and troughs, and to upper-level lows and troughs that are only weakly developed.


Dew

Water condensed onto grass and other objects near the ground, the temperatures of which have fallen below the initial dew point temperature of the surface air, but is still above freezing. Compare with frost.


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 with respect to water.


Discontinuity

A zone with comparatively rapid transition of one or more meteorological elements.


Disturbance

In meteorology, applied rather loosely: (1) any low pressure or cyclone, but usually one that is relatively small in size; (2) an area where weather, wind, pressure, etc., show signs of cyclonic development; (3) any deviation in flow or pressure that is associated with a disturbed state of the weather, i.e., cloudiness and precipitation; and (4) any individual circulatory system within the primary circulation of the atmosphere.


Diurnal

Daily, especially pertaining to a cycle completed within a 24-hour period, and which recurs every 24 hours.


Divergence

The condition that exists when the distribution of winds within a given area is such that there is a net horizontal flow of air outward from the region. In divergence at lower levels, the resulting deficit is compensated for by subsidence of air from aloft; consequently the air is heated and the relative humidity lowered making divergence a warming and drying process. Low-level divergent regions are areas unfavorable to the occurrence of clouds and precipitation. The opposite of convergence.


Doldrums

The equatorial belt of calm or light and variable winds between the two tradewind belts. Compare intertropical convergence zone.


Downdraft

A relative small scale downward current of air; often observed on the lee side of large objects restricting the smooth flow of the air or in precipitation areas in or near cumuliform clouds.


Drifting snow

A type of hydrometeor composed of snow particles picked up from the surface, but carried to a height of less than 6 feet.


Drizzle

A form of precipitation. Very small water drops that appear to float with the air currents while falling in an irregular path (unlike rain, which falls in a comparatively straight path, and unlike fog droplets which remain suspended in the air).


Dropsonde

A radiosonde dropped by parachute from an aircraft to obtain soundings (measurements) of the atmosphere below.


Dry adiabatic lapse rate

The rate of decrease of temperature with height when unsaturated air is lifted adiabatically (due to expansion as it is lifted to lower pressure).


Dry bulb

A name given to an ordinary thermometer used to determine temperature of the air; also used as a contraction for dry-bulb temperature. Compare wet bulb.


Dry-bulb temperature

The temperature of the air.


Dust

A type of lithometeor composed of small earthen particles suspended in the atmosphere.


Dust devil

A small, vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground.


Duster

Same as dust storm—an unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.


Dust storm (also called duster, black blizzard)

An unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.


D-value

Departure of true altitude from pressure altitude; obtained by algebraically subtracting true altitude from pressure altitude; thus it may be plus or minus. On a constant pressure chart, the difference between actual height and standard atmospheric height of a constant pressure surface.


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