Reference

The necessary ingredients for lightning production are known to be an updraft and a mixture of small hail (also called graupel), supercooled water droplets, and ice particles at temperatures colder than freezing (Saunders 1993; Stolzenburg et al. 1998; Rakov 2016). Graupel is a small irregularly shaped ice particle.

Lightning production As the area of negative charge at the base of the thundercloud builds up, it induces a region of positive charge to develop on the ground below. It is the formation and separation of positive and negative electric charges within the atmosphere that creates the highly intensive electric field needed to support this natural spark discharge that is lightning. Once the voltage reaches a certain strength, the air between the base of the cloud and the ground develops an electrical conductivity. The negative charges accumulate at the bottom part of the cloud and the positive charges towards the top. Once connected, electrons from the cloud can flow to the ground and positive charges can flow from the ground to the cloud.

In the initial stages of development, air acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground; however, when the differences in charges becomes too great, this insulating capacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning. A well-developed thunderstorm cloud contains mostly small ice crystals in the upper levels of the cloud, a mixture of small ice crystals and small hail in the middle levels of the cloud, and a mixture of rain and melting hail in the lower levels of the cloud. The negative charge in the middle of thunderstorm cloud causes the ground underneath to become positively charged, and the positively charged anvil causes the ground under the anvil to become negatively charged. If you count the seconds between the flash of lightning and the crack of thunder and divided by 5, you get the number of miles away from you (10 seconds is 2 miles). These collisions and air movements cause the top of the thunderstorm cloud to become positively charged and the middle and lower part of the storm to become negatively charged.