Abstract: Based on Haltiner's columnar model, a stability index WEL containing the process of entrainment and the effect of loading water is presented. The index may show the development process of weaken cumulus.By using Changsha's data of July from 1962 to 1968 the results of test show that the relationship between the stability index WEL calculated by sounding data at eight o'clock and regional thunderstorms or showers in afternoon is satisfactory. The ability of the stability index WEL to distinguish whether thunderstorm and shower occur or not is greater than that of Showalter's stability index. WEL is convenient for operational forecasting use.