When the small water droplets in the cumulonimbus cloud constantly collide and merge into larger water droplets, they begin to fall, while the hot air rising from the ground keeps rushing upwards, and the friction between them is charged. The rising airflow is positively charged and the falling water droplets are negatively charged.

With the passage of time, a large number of positive charges have accumulated at the top of cumulonimbus clouds, while many negative charges have accumulated at the bottom. Due to the negative charge at the bottom of cumulonimbus clouds, the ground is also positively charged.

The water droplets in the cloud merge and increase until the rising hot air can't hold it, and then fall straight from the cloud. The hot air at the lower level was drenched by rain and suddenly became cold. Instead of rushing up, it turned to the ground. At this point, the charge in the air began to discharge, accompanied by thunderous thunder.

Extended data:

Judging from the time of day, thunderstorms mostly occur in the afternoon and evening.

There are more opportunities on land when the thermal convection is strong in summer afternoon, forming a "thermal thunderstorm". At sea, thunderstorms often occur at night because of the slow cooling of the sea surface and unstable air at night.

Thunderstorms near the front can occur simultaneously. In mountainous areas, topographic precipitation can occur. Thunderstorms are usually paroxysmal, and there are often strong winds. Sometimes accompanied by tornadoes and hail. Persistent and intense thunderstorms can often reach the level of heavy rain.

The most active areas of thunderstorms in the world are the tropics and mountains. There are 75- 100 thunderstorm days in the tropics, 20-50 thunderstorm days in the middle latitudes, only a few thunderstorm days in the polar regions, and there are no thunderstorms all the year round north of 82 degrees north latitude.

References:

Baidu encyclopedia-thunderstorm