Stability index

stability index[edit | edit source]

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, T_p and D_p correspond to the temperature and dewpoint at pressure levels p. Those most commonly used are the following.

Showalter stability index: An index given by

where T_L 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).

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.

Totals Indices: The Total Totals index is attributable to Miller (1972). It is defined as the sum of two indices:

where VT is the Vertical Totals index, defined by

Avalue of about 40 corresponds to a dry-adiabatic lapse rate. For a moist- adiabatic lapse rate it is about 20 for T₈₅₀ = 15°C, about 30 for T₈₅₀ = 0°C. The Cross Totals index, CT, is 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.

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 ΔV_{500 - 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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