| Section S | S index | 131-139 of 1376 terms |
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Scorer parameter—The quantity l(z) arising from the wave equation for atmospheric gravity waves describing flow over a mountain barrier:  where N = N(z) is the Brunt–Väisälä frequency and U = U(z) is the vertical profile of the horizontal wind, both quantities determined from an atmospheric sounding upstream of the barrier. The first term on the right-hand side usually dominates, but occasionally the second term, the velocity-profile curvature term, can be of similar magnitude. When l2 is nearly constant with height, conditions are favorable for vertically propagating mountain waves. This parameter is most often used, however, as an indicator of when trapped lee waves (see mountain wave) can be expected; they occur when l2(z) decreases strongly with height. This is especially true if this decrease occurs suddenly in mid troposphere, dividing the troposphere into two regions, a lower layer of large l2(z) (high stability) and an upper layer of small l2(z) (low stability). l, the square root of the parameter, has units of wavenumber (inverse length), and the wavenumber of the resonant lee wave lies between l of the upper layer and l of the lower layer—the equivalent wavelength generally lying between 5 and 25 km in the atmosphere. Mountain ranges wide enough to force wavelengths long relative to lupper (the l in the upper layer) produce vertically propagating waves with wavenumbers less than lupper. Small objects (that force wavenumbers greater than llower) produce waves that are evanescent, or vanishing with height.
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Scotch mist—A combination of thick mist (or fog) and heavy drizzle occurring frequently in Scotland and in parts of England. In the Devon-Cornwall peninsula, the same phenomenon is referred to as “mizzle.”
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scotopic vision—Vision mediated by rods alone at very low levels of luminance. Rods do not allow color vision. See dark adaptation.
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screened pan—An evaporation pan the top of which is covered by wire mesh or screening to prevent animals from drinking the water and debris from falling into it. The screening reduces air circulation and insolation, and increases the pan coefficient.
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screening layer—Space charge layer on the surface of a cloud opposite in polarity to the main charge inside the cloud. This can lead to the underestimation of the magnitude of the main charge when measured remotely.
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scrubber—A flue gas system that removes most of the sulfur dioxide, SO2, from exhaust gases; generally used for large coal-fired boilers. See also limestone scrubbing.
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scud—Ragged low clouds, usually stratus fractus or cumulus fractus, that occur below the main cloud base. They are often found in the vicinity of rainshafts. Several mechanisms may explain their occurrence. They may represent rising air that has greater-than-average humidity due to the evaporation of rain or water on the ground. They may evaporate before reaching the main cloud due to mixing with the intervening dry air. Scud may also be attributed to the breakup of raindrops. The breakup fragments that are able to accumulate in regions of high humidity in rainshafts may be the source of some scud. See pannus.
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