Based on the 634 times of sounding data at 9 o'clock in June,July and August from 1971to 1978 at the hail suppression site in Yongdeng,Gansu province,the classification of weather phenomena and the surface observation were analysed. The results show that the height of 0'C dew-point temperture is an indicaton of halfday and the dew-point depressions at upper levels are of the signification representing the hail intensity. These indicators provide a reference for hail forecast.