K-index (meteorology)

The K-Index or George's Index is a measure of thunderstorm potential in meteorology. According to the National Weather Service, the index harnesses measurements such as "vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer."^[1] It was developed by the American meteorologist J.J. George, and published in the 1960 book Weather Forecasting for Aeronautics.^[2]

Definition

The index is derived arithmetically by:^[3]

$K=(T_{850}-T_{500})+T_{d_{850}}-(T_{700}-T_{d_{700}})$

Where :

T_{d_{850}}

= dew point at 850 hPa

T_{850}

= Temperature at 850 hPa

T_{500}

= Temperature at 500 hPa

Interpretation

The K Index is related to the probability of occurrence of a thunderstorm. It was developed with the idea that Probability = 4 x (K - 15), which gives the following interpretation:^[1]^[4]

K-index values vs. Thunderstorm Probability
K-index value	Thunderstorm Probability
Less than 20	None
20 to 25	Isolated thunderstorms
26 to 30	Widely scattered thunderstorms
31 to 35	Scattered thunderstorms
Above 35	Numerous thunderstorms

References

1 2 "K-Index". weather.gov. National Oceanic and Atmospheric Administration. Archived from the original on July 5, 2015. Retrieved July 5, 2015.
↑ J.J. George (1960). Weather Forecasting for Aeronautics. New York City: Academic Press. p. 673.
↑ Sirvatka. "Stability Indices". Notes de cours. College of DuPage. Retrieved October 30, 2015.
↑ Canadian Meteorological Centre. "Stability Indices". Formation des météorologues. Meteorological Service of Canada. Retrieved October 30, 2015.

Meteorological data and variables

General	Adiabatic processes Advection Buoyancy Lapse rate Lightning Surface solar radiation Surface weather analysis Visibility Vorticity Wind Wind shear

Condensation	Cloud Cloud condensation nuclei (CCN) Fog Convective condensation level (CCL) Lifted condensation level (LCL) Precipitation Water vapor

Convection	Convective available potential energy (CAPE) Convective inhibition (CIN) Convective instability Convective momentum transport Convective temperature (T_c) Equilibrium level (EL) Free convective layer (FCL) Helicity K Index Level of free convection (LFC) Lifted index (LI) Maximum parcel level (MPL) Bulk Richardson number (BRN)

Temperature	Dew point (T_d) Dew point depression Dry-bulb temperature Equivalent temperature (T_e) Forest fire weather index Haines Index Heat index Humidex Humidity Relative humidity (RH) Mixing ratio Potential temperature (θ) Equivalent potential temperature (θ_e) Sea surface temperature (SST) Thermodynamic temperature Vapor pressure Virtual temperature Wet-bulb temperature Wet-bulb potential temperature Wind chill

Pressure	Atmospheric pressure Baroclinity Barotropicity Pressure gradient Pressure-gradient force (PGF)

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