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]


The index is derived arithmetically by:[3]

Where :

= dew point at 850 hPa
= Temperature at 850 hPa
= Temperature at 500 hPa


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


  1. 1 2 "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.
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