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d'Alembert's paradoxA hydrodynamical paradox arising from the neglect of viscosity in the steady flow of a fluid around a submerged solid body. According to this paradox, the submerged body would offer no resistance to the flow of an inviscid fluid and the pressure on the surface of the body would be symmetrically distributed about the body. This paradox may be traced to the neglect of the viscous forces, which are indirectly responsible for fluid resistance by modifying the velocity field close to a solid body.
Birkhoff, G., 1950: Hydrodynamics, 10–22.
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D-regionThe lowest region of the ionosphere. The term is used somewhat loosely to describe the ionization, beginning about 70 km and merging with the E-region, that does not usually produce an echo on normal ionosonde recordings. The main effect of the D-region on radio waves is one of absorption, thus inhibiting long- distance propagation of HF and VHF radio waves in daytime, when D-region ionization is most intense. At low and middle latitudes, the D-region is produced mainly by the action of solar radiation on nitric oxide (NO). At high magnetic latitudes energetic particles of solar or auroral origin may be the principal source, in which case radio waves can be strongly absorbed at all times of day. The term D-layer is used occasionally by analogy with the higher E- and F-layers, which produce sharply defined echoes on ionosonde recordings.
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dadurIn India, a wind blowing down the Ganges Valley from the Siwalik hills at Hardwar.
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daily forecast(Sometimes called short-range forecast.) A forecast for the ordinary daily range, for periods of from 12 to 48 hours in advance. Forecasts of this type usually express in some detail, for specific geographical areas, the expected day-to-day sequence of all the aspects of weather that materially affect human activity and well- being. See short-range forecast; compare medium-range forecast, long-range forecast, extended forecast.
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daily maximum temperatureMaximum temperature in the course of a continuous time interval of 24 hours (usually midnight to midnight local time).
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daily meanThe long-period mean value of a climatic element on a given day of the year. A curve of daily means throughout the year shows the annual variation in much greater detail than a curve based on monthly means, but unless it is based on a long period (at least 50 years) it will probably be dominated by random irregularities. Such long-period daily means might be referred to as “normal daily values.”
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