| Section R | R index | 391-399 of 589 terms |
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relative sea level—Sea level measured relative to a local tide gauge benchmark. Changes include both local vertical land movements and absolute sea level changes.
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relative sunspot number—(Also called sunspot number, Wolf number, Zurich number.) A measure of sunspot activity, computed from the formula R = k(10g + f), where R is the relative sunspot number, f the number of individual spots, g the number of groups of spots, and k a factor that varies with the observer's personal equation, the seeing, and the observatory (location and instrumentation).
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relative vorticity—(Also called local vorticity.) The vorticity as measured in a system of coordinates fixed on the earth's surface. Usually, only the vertical component of the vorticity is meant. See also geostrophic vorticity, vorticity equation.
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relaxation method—An iterative numerical method for solving elliptic partial differential equations. For example, a Poisson equation, ∇2φ = F(x, y), where ∇2 is the Laplacian operator and the function F(x, y) is given.
Panofsky, H., 1956: Introduction to Dynamic Meteorology, 168–174.
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relaxation of a trough—A decrease in the amplitude of a trough. In middle latitudes, it implies a poleward movement of an isopleth that defines a trough (i.e., height or thickness contour). It is the opposite of meridional or equatorward extension of a trough.
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relaxation time—In general, the time interval required for a system exposed to some discontinuous change of environment to undergo the fraction (1 − e−1), or about 63%, of the total change of state that it would exhibit after an infinitely long time. For example, a thermometer initially at equilibrium in a bath at temperature T1 will exhibit an exponential change of temperature with time after being suddenly plunged into a bath at temperature T2, theoretically assuming the new temperature T2 only after an infinitely long time. The finite time interval required for the thermometer to undergo a change of amount (T1 − T2)(1 − e−1) is called the thermal relaxation time of the thermometer. Occasionally, the fraction 9/10 is used in place of (1 − e−1), so contexts must always be checked to be certain of the definition employed in a given case. The definition may also change for an underdamped device. The change of state of such a device may oscillate several times while approaching its final value.
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relaxed eddy accumulation—Method of measuring fluxes of trace gases; similar to eddy accumulation except that the volume rate at which the sample is collected is constant. The flux is estimated as the product of the standard deviation of the vertical velocity, the difference in the concentration in each reservoir, and a scaling parameter. The technique has been applied to the measurement of the fluxes of heat, water vapor and some chemical species.
Businger, J. A., and S. P. Oncley, 1990: Flux measurement with conditional sampling. J. of Atmos. Oceanic Technol., 7, 349–352.
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