| Section A | A index | 641-649 of 917 terms |
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apogean winds—Land winds of Greece.
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apogee—The point in an orbit at which any orbiting object, for example, planet, moon, artificial satellite, is farthest from the central object. Opposite of perigee. Compare perihelion.
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apparent force—(Also fictitious force, inertial force, transport force.) A force (mass times an acceleration) introduced on the side of an equation on which all supposedly real forces appear. For Newton's dynamical law of motion for a body of mass m acted on by a force F,  to be valid requires that the acceleration a be specified relative to an inertial reference frame. If the acceleration in a noninertial reference frame (e.g., a rotating reference frame) is a*, then  and the previous dynamical equation may be written  where the quantity − mai is the inertial or apparent force and ai is the inertial acceleration. Examples of apparent forces are the centrifugal force and the Coriolis force. Within classical (Newtonian) mechanics inertial forces are fictitious, merely masses times accelerations. But in general relativity, inertial forces are equivalent to real forces resulting from interactions between bodies because it is impossible to distinguish between inertial and gravitational accelerations; both are independent of the mass of the body. See apparent gravity.
Symon, K. R., 1960: Mechanics, 2d ed., p. 271.
Bergmann, P. G., 1942: Introduction to the Theory of Relativity, 155–156.
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apparent gravity—(Also called effective gravity, virtual gravity.) For a rotating planet such as Earth the resultant of the force (per unit mass) due to gravity and the centrifugal force. Apparent gravity is given by  where g is the force (per unit mass) due to gravitational attraction, Ω is the angular velocity of the planet, and r is the position vector relative to its center. Like g, g* depends only on position. Within the earth's atmosphere the magnitude of the centrifugal force is less than 0.03% of g. See apparent force.
Dutton, J. A., 1976: The Ceaseless Wind, p. 225.
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apparent groundwater velocity—(Also called Darcy velocity.) The volume of groundwater passing through a unit area of porous medium, perpendicular to the direction of flow, in a unit of time.
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apparent horizon—The line between the sky and the earth projected on the celestial sphere. This line is often irregular, due to the earth's topography. On flat surfaces, such as the sea or a level plane, the line approximates a great circle if the observer's elevation above the surface is insignificant and a small circle if the elevation is appreciable.
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apparent solar day—(Also called true solar day.) The interval of time between two successive transits of the sun across a meridian. This interval is about four minutes longer than the sidereal day, largely because of the sun's apparent annual motion eastward along the ecliptic (actually, the earth's “westward” motion along its orbit), which motion delays the sun's return to meridional transit. Also, this interval is inconveniently nonuniform due to systematic variations in the earth's orbital speed around the sun and the sun's changing declination. The concept of the mean solar day has been invented to circumvent these practical difficulties.
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apparent stress—A process that has the same effect as a stress. An example is the Reynolds stress, where turbulent motions can exchange fluid from within a hypothetical cube of air with fluid of different velocity from outside the cube, causing the cube to deform in a manner similar to a stress acting on the face of the cube.
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Applications Technology Satellite—(Abbreviated ATS.) The first experimental geostationary satellite series, operated by NASA, that preceded the SMS and GOES series. ATS-1, launched in December 1966, carried the first spin scan cloud camera, providing full- disk visible imagery every 30 minutes, and introduced WEFAX data relay.
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applied climatology—The scientific analysis of climatic data in the light of a useful application for an operational purpose. “Operational” is interpreted as any specialized endeavor within such as industrial, manufacturing, agricultural, or technological pursuits (after Landsberg and Jacobs 1951). This is the general term for all such work and includes agricultural climatology, aviation climatology, bioclimatology, industrial climatology, and others.
Landsberg, H. E. and Jacobs, W. C., 1951: Compendium of Meteorology, 976–992.
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