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O'Brien cubic polynomial—An approximation for the eddy diffusivity K as a function of height z in a boundary layer of depth h with surface layer (SL) of depth zSL:  where the tunable parameters in the equation are eddy diffusivities at the top of the surface layer and at the top of the boundary layer, K(zSL) and K(h), respectively, the heights of those two layers, and the gradient of eddy diffusivity at the top of the surface layer ∂K/∂z. Above the top of the boundary layer the eddy diffusivity is assumed to be constant at K(h), while at the surface it is assumed to be zero. See K-theory, gradient transport theory, first-order closure, closure assumptions.
O'Brien, J. J., 1970: A note on the vertical structure of the eddy exchange coefficient in the planetary boundary layer. J. Atmos. Sci., 27, 1213–1215.
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oasis effect—Evaporative cooling effect due to heat advection when a source of water exists in an otherwise arid area. In addition to true desert oases, the oasis effect is also characteristic of natural bodies of water in arid surroundings, melting snow patches, irrigated fields in arid areas, or irrigated urban lawns and parks. Latent heat flux from such an oasis can exceed the locally available radiative flux twofold; advection of sensible heat from surrounding warmer surfaces and airmass subsidence over the cooler area provides the remainder. Evaporation also exceeds the local precipitation, the extra water coming from wells, river flow, and irrigation.
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oberwind—A night wind from mountains or the upper ends of lakes; a wind of Salzkammergut in Austria.
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objective analysis—An analysis that is free from any direct subjective influences resulting from human experience, interpretation, or bias. See objective forecast.
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oblique visual range—(Also called oblique visibility, slant visibility.) The greatest distance at which a specified target can be perceived when viewed along a line of sight inclined to the horizontal. One must distinguish upward from downward oblique visual range because of the quite different background luminance prevailing in the two cases. Furthermore, a range can only be considered with respect to some given type of target, as is also true of the ordinary (horizontal) visual range. In view of the great importance of the downward oblique visual range in air to ground visual contact and the upward oblique visual range in visual detection of aircraft, it is unfortunate that no satisfactory theory for this has yet been developed. The principal obstacle in treating this problem lies in the typically nonuniform height variation of the extinction coefficient.
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obliquity of the ecliptic—The angle between the plane of the ecliptic (or the plane of the earth's orbit) and the plane of the earth's equator; the “tilt” of the earth. The obliquity of the ecliptic is computed from the following formula:  where t is the year for which the obliquity is desired. For 1999, the value was 23°26′21.89″. It is the oblique orientation of the earth's axis relative to its orbit that accounts for the seasons, for, in the period of a year, the angle of incidence of incoming solar radiation varies by nearly 47° at any one place. Particularly at high latitudes, this results in a great seasonal temperature contrast. M. Milankovitch has calculated that the obliquity of the ecliptic varies between 24.5° and 22° in the course of 40 000 years. This variation may be considered as a long-period climatic control and is included in the astronomical theory of ice ages.
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