K Index (George's Thunderstorm Index)

Three terms, three jobs. The K index is worked out from readings of a weather-balloon sounding to gauge the chance of air-mass thunderstorms: $K = (T_{850} - T_{500}) + T_{d,850} - (T_{700} - T_{d,700})$. The first bracket is the cooling between 850 and 500 hPa, which measures how steeply the air cools with height. The second term, the dew point at 850 hPa, measures the moisture low down. The last bracket is the dew-point depression at 700 hPa, how far the temperature stands above the dew point there, which measures how deep the moist layer is. It is subtracted, so a small depression, meaning moist air at that level, raises the index.

Reading the answer. The index is itself read in degrees. Below 20 no thunderstorms are expected; 20 to 25 means isolated ones, 26 to 30 widely scattered, 31 to 35 scattered, and above 35 numerous. You can enter each temperature in any unit, but the sum is done in degrees Celsius. That matters here because the 850 hPa dew point enters on its own rather than as a difference, so the zero of the temperature scale would otherwise change the answer.

A worked case. Take 20, 14, 8, 2 and minus 12 °C for $T_{850}$, $T_{d,850}$, $T_{700}$, $T_{d,700}$ and $T_{500}$. The cooling bracket is 32, the 850 hPa dew point adds 14, and the 700 hPa depression takes away 6, so $K = 32 + 14 - 6 = 40$: numerous thunderstorms. A cooler, drier sounding of 12, 8, 2, minus 4 and minus 18 °C gives $30 + 8 - 6 = 32$: scattered. The page solves for any one reading, so $K = 40$ with the other four readings returns $T_{d,850} = 14$ °C, or a $T_{500}$ of minus 12 °C.

Where it comes from. The index is from George's Weather Forecasting for Aeronautics (1960), and the page's definition matches the National Weather Service glossary entry for the K-index. It uses only the 850, 700 and 500 hPa levels. The total-totals index is built from the same 850 and 500 hPa readings, the lapse-rate page covers the cooling that the first bracket measures, and the dew-point page gives a dew point from a temperature and a humidity when a station reports only those.