K-index (meteorology)

Lightning

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 Joseph J. George, and published in the 1960 book Weather Forecasting for Aeronautics.^[2]

Definition

The index is derived arithmetically by:^[3]

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

Where :

${\displaystyle T_{d_{850}}}$ = Dew point at 850 hPa

${\displaystyle T_{850}}$ = Temperature at 850 hPa

${\displaystyle T_{d_{700}}}$ = Dew point at 700 hPa

${\displaystyle T_{700}}$ = Temperature at 700 hPa

${\displaystyle T_{500}}$ = 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 Potential = 4 x (KI - 15), which gives the following interpretation:^[1][4]

K-index values vs. Thunderstorm Probability
K-index value (in k)	Thunderstorm Probability
Less than 20	None
20 to 25	Isolated thunderstorms
26 to 30	Widely scattered thunderstorms
31 to 35	Scattered thunderstorms
35 - 60	Numerous thunderstorms
60 - 100	Thunderstorm
100	Hailstorm

References

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