_Deperditometer._--An instrument devised by A. Piche for measuring the cooling power of the atmosphere, with reference to its physiological effects. _Depression._--A cyclonic area, or low. _Desert Sounds._--(See _brontide_.) _Devil._--The name applied to a dust whirlwind in India. The term is also current in South Africa. _Dew._--Atmospheric moisture condensed, in liquid form, upon objects cooler than the air, especially at night. _Dew-point._--The temperature at which, under ordinary conditions, condensation of water vapor begins in a cooling mass of air. It varies with the absolute humidity. _Dew Pond._--The name applied in southern England to certain artificial ponds on the uplands. They contain water in the driest weather, and are popularly supposed to be fed by dew. “_Doctor._”--A colloquial name for the sea breeze in tropical climates. The name is sometimes applied to other cool, invigorating breezes. _Doldrums._--The equatorial belt of calms or light, variable winds, lying between the two trade-wind belts. _Drought._--A protracted period of dry weather. In the United States a drought has been defined as a period of thirty or more consecutive days during which precipitation to the amount of 0.25 inch does not occur in twenty-four hours. Other quantitative definitions have been used in other countries. _Dry Fog._--A haze due to the presence of dust or smoke in the air. _Dust-counter._--An instrument for determining approximately the number of dust particles or condensation nuclei per unit volume in a sample of air. _Dynamic Meteorology._--The branch of meteorology that treats of the motions of the atmosphere and their relations to other meteorological phenomena. _Earth-air Current._--The electrical current that passes between the earth and the air on account of their difference of potential. _Eddy._--A more or less fully developed vortex in the atmosphere, constituting a local irregularity in a wind. All winds near the earth’s surface contain eddies, which at any given place, produce “gusts” and “lulls.” Air containing numerous eddies is said to be “turbulent.” _Electric Niagara._--(See _hail rod_.) _Evaporimeter._--(See _atmometer_.) _Eye of the Storm._--A calm region at the center of a tropical cyclone, or a break in the clouds marking its location. _Fall Wind._--A wind blowing down a mountain-side; or any wind having a strong downward component. Fall Winds include the foehn, mistral, bora, etc. _False Cirrus._--Cirruslike clouds at the summit of a thundercloud; probably identical in structure with true cirrus, or cirro-stratus. Sometimes more appropriately called “thunderstorm cirrus.” _Fata Morgana._--A complex form of mirage, characterized by marked distortion of images. _Festoon Cloud._--Mammato-cumulus.