In many places in Henan Province, the hailstones turned white instantly. Why is this extreme weather?

In many places in Henan Province, the ground under hail is suddenly white, and the formation of this extreme weather is related to the local topography, climate and the weather at that time. Hail weather is a strong convective weather.

The convective system causing hail has the characteristics of small spatial scale, rapid development, short life and great harm. Hail is born in the development of powerful cumulonimbus clouds, which are called hail clouds. Hail clouds are composed of water droplets, ice crystals and snowflakes, which are usually divided into three layers: the bottom layer is composed of water droplets with a temperature above 0℃; The second layer consists of water droplets with a temperature higher than 0℃. The temperature in the middle layer is -20? 0℃, composed of supercooled water droplets, ice crystals and snowflakes. The temperature of the top layer is lower than -20℃, which is basically composed of ice crystals and snowflakes.

Large-scale circulation and environmental conditions conducive to the development of convection system are the basis of strong convective weather. The formation of hail is firstly the nucleation and propagation of water vapor to produce ice crystals, and then the ice crystals and snow are transformed into hail. Ice droplets are formed by freezing or nucleation of supercooled raindrops when they contact with ice crystals and snow. However, the former is far greater than the latter in quantity and quality. The growth of frozen drops is mainly to collect supercooled rainwater first, and then collect cloud water. The conversion rate of graupel hail is lower than that of frozen drop to hail. Therefore, the embryo of hail is mainly frozen water droplets, and the growth of hail is mainly frozen cold cloud water. Suitable ice crystals, snow and a lot of supercooled water, especially in the early stage of hail cloud development, are very beneficial to the formation and growth of hail.

The convergence line of ground front, dew point front and other boundary layers provides a trigger mechanism for the occurrence and development of convection system. The dry cold air carried by westerly jet in the middle and upper floors is superimposed on the humid and hot air in the lower floors in the south. Coupled with the uneven heating of the underlying surface, strong thermal instability stratification is formed in the local terrain, which is beneficial to the development and enhancement of the convection system. When the system is forward-leaning structure, it is more conducive to triggering a wide range of hail disaster weather.