The cold air has ice crystals. The warm air has water droplets. During the storm, the droplets and crystals bump together and move apart in the air. This rubbing makes static electrical charges in the clouds.

How do clouds become electrically charged?

Air molecules and suspended water droplets collide as they swirl around in the clouds. Warmer air and water droplets rise, carrying charges with them. The result is an excess of positive charge near the cloud tops, and an excess of negative charge in the bottom layers of the clouds. ... —to become positively charged.

What causes cloud thunder?

Thunder is caused by the rapid expansion of the air surrounding the path of a lightning bolt. ... As lightning connects to the ground from the clouds, a second stroke of lightning will return from the ground to the clouds, following the same channel as the first strike.