An abrupt heavy rain event occurred in Wuhan from 20 to 22 July 1998 is analyzed by using conventional and satellite data. The results show that: (1) The event was caused by a meso βscale convective cloud cluster. (2) The meso β scale system is not related with strong cyclone vorticity or ant cyclone vorticity center in lower or upper troposphere. The divergence strength of the system is higher 10¹s^-1 than that of vorticity. (3) 10 hours before its occurrence, the atmospheric motion was already unbalanced. After the precipitation, its intensity got strongest and it changed into U >0 from U <0. (4) The unbalanced force of lower atmospheric motion and strong divergence in the upper atmosphere are led to the initiation and development of the meso-βscale system as well as the occurrence of heavy rain.