| Section D | D index | 371-379 of 573 terms |
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diurnal—Daily, especially pertaining to actions that are completed within 24 hours and that recur every 24 hours; thus, most reference is made to diurnal cycles, variations, ranges, maxima, etc. The diurnal variability of nearly all of the meteorological elements is one of the most striking and consistent features of the study of weather. The diurnal variations of important elements at the earth's surface can be summarized as follows: 1) temperature maximum occurs after local noon and minimum near sunrise; 2) relative humidity and fog are the reverse of temperature; 3) wind generally increases and veers by day and decreases and backs by night (see heliotropic wind, land and sea breeze, mountain and valley wind); 4) cloudiness and precipitation over a land surface increase by day and decrease at night; over water the reverse is true, but to a lesser extent; 5) evaporation is markedly greater by day; 6) condensation is much greater at night; 7) atmospheric pressure varies diurnally or semidiurnally according to the effects of atmospheric tides.
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divergence line—(Sometimes called asymptote of divergence.) Any horizontal line along which horizontal divergence of the airflow is occurring.
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divergence signature—A pattern in a Doppler velocity field representing horizontal divergence (or convergence) associated with atmospheric phenomena. In a storm-relative reference frame, the idealized signature associated with flow diverging from a point source (or converging toward a sink) is symmetric about a line perpendicular to the radar viewing direction with marked radial shear across the core region between peak Doppler velocity values of opposite sign. The signature associated with a divergence line (or convergence line) depends on the Doppler radar viewing direction. When the radar viewing direction is perpendicular to the line, there is a band of radial shear across the line that corresponds to the full measure of divergence (convergence). When the viewing direction is parallel to the line, none of the divergence is sensed by the radar. At angles in between, a fraction of the divergence is sensed.
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divergence theorem—(Also called Gauss's theorem.) The statement that the volume integral of the divergence of a vector, such as the velocity V, over a volume V is equal to the surface integral of the normal component of V over the surface s of the volume (often called the “export” through the closed surface), provided that V and its derivatives are continuous and single-valued throughout V and s. This may be written  where n is a unit vector normal to the element of surface ds, and the symbol ∮ ∮S indicates that the integration is to be carried out over a closed surface. This theorem is sometimes called Green's theorem in the plane for the case of two-dimensional flow, and Green's theorem in space for the three-dimensional case described above. The divergence theorem is used extensively in manipulating the meteorological equations of motion.
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divergence—The expansion or spreading out of a vector field; also, a precise measure thereof. In mathematical discussion, divergence is taken to include convergence, that is, negative divergence. The mean divergence of a field F within a volume is equal to the net penetration of the vectors F through the surface bounding the volume (see divergence theorem). The divergence is invariant with respect to coordinate transformations and may be written  where ∇ is the del operator. In Cartesian coordinates, if F has components Fx, Fy, Fz, the divergence is  Expansions in other coordinate systems may be found in any text on vector analysis. In hydrodynamics, if the vector field is unspecified, the divergence usually refers to the divergence of the velocity field (see also mass divergence). In meteorology, because of the predominance of horizontal motions, the divergence usually refers to the two-dimensional horizontal divergence of the velocity field  where u and v are the x and y components of the velocity, respectively. This divergence is denoted by any of the following symbols:  where the last two quantities involve derivatives in the isobaric surface. The order of magnitude of the horizontal divergence in meteorological motions is of considerable dynamic importance: The geostrophic wind has divergence of the order of 10−6s−1; the wind field associated with migratory cyclonic systems, 10−5s−1; motions of smaller scale (such as gravity waves, frontal waves, and cumulus convection) have characteristic divergence one or two orders of magnitude greater. See balance equation, deformation; compare diffluence, curl, vorticity.
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diversion of water—Transfer of water from one watercourse to another, such watercourses being either natural or man-made.
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dividing streamline—A streamline or flow boundary representing the height that separates stable flow approaching a hill or mountain into two regions: a lower region, where the flow is horizontal and splits around the obstacle (i.e., the flow is partially blocked), and an upper region, where the flow is three-dimensional and goes over the hill. This occurs when the relationship between the flow and the obstacle height H is characterized by a Froude number Fr less than 1. The height of the boundary Hc has been found to be Hc = H(1 − Fr ). Thus, it lies between the top and bottom of the hill and is lower for more stable flow. This concept has been used in air pollution studies to determine where pollution plumes are apt to impinge on a slope surface.
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