Affected by typhoon residual circulation and weak cold air in hail clouds, the updraft is changeable. When the updraft is strong, the water droplets in the lower part of the cloud are brought to the upper and middle layers of the cloud, and the water droplets quickly cool and solidify into small ice crystals. In the process of falling, small ice crystals collide with supercooled water droplets to form hail embryos. However, there are many typhoons in Hainan, and hail is generally caused by typhoon residual circulation and weak cold air. Hail occurs 3~4 times a year in Hainan Island, mostly in March ~ May. Because in this season, the atmosphere often appears unstable stratification with cold upper and warm lower, dry upper and wet lower.