What Causes Lightning?
Lightning is an electric current. During a thunderstorm, water droplets in the clouds become charged with electricity as they are bounced about in the swirling air. After a while, the whole cloud fills up with electrical charges.
Positive and negative electrical charges build up in the clouds. The positive charges or protons form at the top of the cloud and the negative charges or electrons form at the bottom of the cloud.
When the electrical charges in a cloud become strong enough, a huge spark or flash of electricity jumps between them. The electrical discharge may jump between two parts of the same cloud, between two clouds, or between a cloud and the earth.
The electricity heats the air along the path of the flash, causing the air to glow for an instant, and we see a lightning bolt.