From Glossary of Meteorology
An index indicating the initiation of convective cloud.
This is defined as
where sfc is the surface elevation of the station in m; T850, T700, and T500 are the 850-, 700-, and 500-hPa temperatures in °C; and PW700 is the precipitable water in cm between the surface and the 700-hPa level. The more negative the index, the greater the probability of convective precipitation.