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.
Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact email@example.com. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.