Quote:
Originally Posted by Ref Ump Welsch
If you knew your weather facts, lightning does not come from the sky, it comes from the ground. The charges are at both ends, sky and ground, but it originates in the ground.
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This is a little misleading I think.
Storm clouds in general become charged with a high concentration of negative charge at the bottom of the cloud. The ground builds up positive charge underneath the cloud (since opposite charges attract). As a result, there is an electric field between the ground and earth that can be quite substantial.
The air between the cloud and earth is initially non-conductive. Large electric fields can ionize the air creating positively charged ions and free electrons that make the air conductive.
Before lightning strikes, the air beneath the clouds starts to ionize forming what is called a step leader that attempts to find the path of least resistance towards the ground. As a result of this negative charged step leader, the ground becomes more positively charged underneath this step leader. The area is about the size of a football field and it is not evenly charged due to the surface being nonhomogeneous and the ground being resistive. The positive charge begins to rise from the ground forming streamers from areas of the ground with higher elevation (trees, buildings, people (on a football field with no trees or buildings), bleachers, etc.). Once contact is made between the leader and the streamers, you have lighting (caused by a large current ionizing the air around it) and thunder (caused by the air heating up and exploding).
The current (flows positive to negative) flows from the ground to the cloud. But it is the negative cloud that causes this large charge buildup in the ground.