| Section S | S index | 861-869 of 1376 terms |
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St. Swithin's Day—In English folklore, a day that is popularly supposed to govern the weather of the succeeding 40 days; specifically, if it rains on St. Swithin's Day, 15 July, it will continue to rain for 40 days. Not even a tendency toward this is borne out by weather records. Similar legends relate to other saints' days in many countries. See control day.
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stability categories—The range of stabilities organized in categories designed to provide semiquantitative measures of the mixing capabilities of the lower atmosphere; usually termed the Pasquill– Gifford categories.
Pasquill, F., 1962: Atmospheric Diffusion, Van Nostrande, p. 209.
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stability index—Any of several quantities that attempt to evaluate the potential for convective storm activity and that may be readily evaluated from operational sounding data. In the definitions, Tp and Dp correspond to the temperature and dewpoint at pressure levels p. Those most commonly used are the following. K-index: This index is due to George (1960) and is defined by  The first term is a lapse rate term, while the second and third are related to the moisture between 850 and 700 mb, and are strongly influenced by the 700-mb temperature–dewpoint spread. As this index increases from a value of 20 or so, the likelihood of showers and thunderstorms is expected to increase. |
Lifted index: This index, developed by Galway (1956), is  so is nominally identical to the Showalter index, except that the parcel being lifted (dry- adiabatically to saturation and then moist-adiabatically to 500 mb) is defined by the dry adiabat running through the predicted surface afternoon temperature maximum and the mean mixing ratio in the lowest 900 m of the sounding. If no further heating is expected, as with a sounding taken in the late afternoon, then the mean potential temperature in the lowest 900 m of the sounding defines the dry adiabat used for the parcel. Numerous variations, focused on how the lifted parcel is defined, have been used since the original definition. The values of this index tend to be somewhat lower than those of Showalter, and the interpretation depends to some extent on how the lifted parcel is defined. |
SWEAT index: (Or severe weather threat index; also abbreviated SWI.) Another index attributable to Miller (1972), used mainly for analyzing the potential for severe thunderstorms, is defined as  where TT is the Total Totals index (set to zero if less than 49), V850 and V500 are the 850- and 500-mb wind speeds, and ΔV500 − 850 is the 500-mb wind direction minus the 850-mb wind direction, in degrees. The last term is set to zero if any of the following conditions are not met: 1) 850-mb wind direction is in the range from 130 to 250 degrees; 2) 500- mb wind direction is in the range 210 to 310 degrees; 3) the difference in wind directions is positive, or 4)both 850- and 500-mb wind speeds are at least 15 knots. No term in the formula is allowed to be negative. The severe thunderstorm threat is considered to increase from values of about 300 and higher; tornadoes are considered to increase in likelihoodfrom values of about 400 and up. |
Showalter, A. K., 1947: A stability index for forecasting thunderstorms. Bull. Amer. Meteor. Soc., 34, 250– 252.
George, J. J., 1960: Weather Forecasting for Aeronautics, Academic Press, 673 pp.
Miller, R. C., 1972: Notes on analysisand severe storm forecasting procedures of the Air Force Global Weather Central. Tech. Rept. 200(R), Headquarters, Air Weather Service, USAF, 190 pp.
Galway, J. G., 1956: The lifted index as a predictor of latent instability. Bull. Amer. Meteor. Soc., 528–529.
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stability—1. The characteristic of a system if sufficiently small disturbances have only small effects, either decreasing in amplitude or oscillating periodically; it is asymptotically stable if the effect of small disturbances vanishes for long time periods. A system that is not stable is referred to as unstable, for which small disturbances may lead to large effects. Some authors also distinguish a neutral or marginally stable case, in which disturbances do not vanish, but also do not grow without bound. Classically, stability was defined only with respect to systems in equilibrium. More recently it has been extended to apply to evolving systems, for which an unstable disturbance leads to an evolution that becomes uncorrelated with the undisturbed evolution. From this standpoint stability and predictability can be equated. 2. Same as static stability. 3. The property that each computed solution (in exact arithmetic) of a finite difference approximation remains bounded for all possible choices of the time step. See Lax equivalence theorem. 4. The ability of laminar flow to become turbulent in a fluid.
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where TL is the temperature (°C) of a parcel lifted from 850 to 500 mb, dry-adiabatically to saturation and moist-adiabatically above that. As the index decreases to zero andbelow, the likelihood of showers and thunderstorms is considered to increase (Showalter 1947).
where VT is the Vertical Totals index, defined by
so is strongly influenced by the 850-mb moisture. Showers and thunderstorms become increasingly likely from TT values of about 30, and severe thunderstorms are considered likely for values of 50 or more.