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Section S S index 291-299 of 1376 terms
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sferics receiver—(Also called lightning direction finder.) An instrument that measures, electronically, the direction of arrival, intensity, and rate of occurrence of atmospherics; a type of radio direction finder, it is most commonly used to detect and locate cloud-to-ground lightning discharges from distant thunderstorms. In its simplest form the instrument consists of two orthogonally crossed antennas that measure the electromagnetic field changes produced by a lightning discharge and determine the direction from which the changes arrived. Negative and positive polarity cloud-to-ground discharges can be distinguished. Cloud-to-cloud discharges can be distinguished based on characteristics of the received signal, and the geometry of nearby discharge channels may be determined. See also narrow- sector recorder, lightning detection network.
sferics source—That portion of a lightning discharge that radiates strongly in the frequency interval 10–30 kHz. The physical source is generally identified with the return stroke in flashes to ground and the K change in the case of intracloud flashes.
sferics—1. (Also spelled spherics.) The study of atmospherics, especially from a meteorological point of view. This involves techniques of locating and tracking atmospherics sources and evaluating received signals (waveform, frequency, etc.) in terms of source. 2. Same as atmospherics.
shadow band pyranometer—An instrument for measuring the diffuse sky radiation. A shadow band is added to a conventional pyranometer at such an angle that it blocks out the direct solar radiation throughout the course of a day.
shadow bands—1. Same as crepuscular rays. 2. A scintillation phenomenon wherein dark bands appear to move rapidly across the luminous surface of any distant light source.
shadow zone—(Often called diffraction zone.) May be used for any region that would not be illuminated by a given source of electromagnetic (or acoustic) radiation if it propagated strictly according to ray optics in a homogeneous (on the scale of the wavelength) medium. Some radiation, however, does penetrate shadow zones because of scattering by the propagating medium or by obstacles within it. Because of the widespread misconception that diffraction is fundamentally different from scattering, the term diffraction zone is also used, especially by radio engineers, who might say that the earth diffracts radio waves into this zone (and probably would ignore scattering by the atmosphere). Yet all radiation that penetrates a shadow zone does so because of scattering by the atmosphere and by solid and liquid bodies (including the earth) within the atmosphere or at its boundary.
shadow—In radar, an azimuth sector with no echoes because the transmitted signal is masked by local prominences such as hills or buildings.
shaitan—Same as dust whirl.
shale ice—Mass of thin and brittle plates of freshwater ice formed by breaking up of thin, skin ice into small pieces lumped together.
shallow convection parameterization—The representation in a numerical model of the turbulent transports of heat and moisture by nonprecipitating cumulus clouds with cloud tops below 3000 m above the surface. These shallow cumulus clouds are found all over the globe but they, and their associated turbulent transports, are of particular significance in the trade wind region where they provide the heat and moisture that maintains the thermodynamic structure of the lower troposphere. See parameterization.
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