A Convective Index as an Indicator of Cumulonimbus Development

Abstract

Empirical methods are used to determine a modification to Jefferson's index of instability which has a good correlation with the incidence and development of cumulonimbus clouds over Edmonton, Alberta. This results in a convective index combining a representative afternoon dew point, the maximum temperature for the day and the temperature at the 500-mb level at the time of maximum heating. A critical value for the incidence of cumulonimbus evolves. Field testing shows increased accuracy in cumulonimbus forecasting when a forecast index is used. The forecast index has been applied to other localities in western Canada and proved to be a useful tool in delineating areas of expected instability from the surface through the 500-mb level.