| Section S | S index | 381-389 of 1376 terms |
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sigma vertical coordinate—A vertical coordinate for atmospheric models defined as pressure normalized by its surface value, or as the difference in pressure and its value at the top of the model atmosphere normalized by the surface value of this difference. Thus, σ = (p − pT)/(pS − pT) where p is pressure, and the subscripts T and S stand for the top and the ground surface values of the model atmosphere, respectively. With the sigma coordinate, the lowest coordinate surface follows the model terrain, resulting in significant simplification of the equations compared to pressure or to an unmodified geometric height coordinate. See eta vertical coordinate, hybrid vertical coordinate, isentropic vertical coordinate, pressure vertical coordinates, height vertical coordinate.
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SIGMET information—(Contraction for significant meteorological information.) Information issued by a meteorological watch office concerning the occurrence or expected occurrence of en route phenomena that may affect the safety of aircraft operations, such as convection, turbulence, and icing.
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signal generator—An electronic instrument used for the production of electromagnetic or acoustic signals with certain desired characteristics. It is useful in testing and calibration.
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signal strength—1. In radio, a measure of the received radio frequency power; generally expressed in decibels relative to some standard value, normally either one milliwatt or that power that would have resulted at the same distance under free space transmission. Could also be applied to the strength of an optical signal transmitted or received by a lidar or an acoustic signal transmitted or received by a sodar. The term field strength is commonly used as a synonym. 2. In radar, the strength of the received signal, usually expressed in logarithmic power units, in particular decibels relative to 1 mW, that is, dBm.
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signal-to-noise ratio—(Abbreviated SNR.) A ratio that measures the information content of a signal, usually defined as the ratio of the power of the signal unaffected by noise to the power of the noise. Commonly measured in decibels, the signal-to-noise ratio is sometimes defined for a specified bandwidth.
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signal velocity—The speed of propagation of a hydrodynamic influence. For the cyclonic-scale quasigeostrophic disturbances, the effective signal velocity is given approximately by the maximum group velocity and is usually of the order of 30° longitude per day.
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