A couple of days ago our car was moving through clouds (yeah, it was very cool!). My family was very worried about the dark cloud. They wondered if lightning can occur in the cloud. This got me thinking how lightning forms. I think this post answers my question.
Lightning begins as static charges in a rain cloud. Winds inside the cloud are very turbulent. Water droplets in the bottom part of the cloud are caught in the updrafts and lifted to great heights where they freeze. Meanwhile, downdrafts in the cloud push ice and hail down from the top of the cloud. When the ice going down meets the water coming up, electrons are stripped off.
The result of this activity is a cloud with a negatively charged bottom and a positively charged top. These electrical fields become incredibly strong, with the atmosphere acting as an insulator between them in the cloud.When the strength of the charge overpowers the insulating properties of the atmosphere, lightning happens.
As the storm moves over the ground, the strong negative charge in the cloud attracts the positive charges in the ground. These positive charges move up into the tallest objects like trees, telephone poles, and houses. As the negative charge gets close to the ground, a positive charge, called a streamer, reaches up to meet the negative charge. The channels connect and we observe the lightning stroke.
This infographis provides some interesting info about lightning.