Lightning is like a giant natural circuit breaker. When the balance in the atmosphere's natural electrical charge becomes overloaded, lightning is what flips nature's switch and restores the balance. These bolts of electricity, which emerge from clouds during thunderstorms, can be dramatic and deadly.
As atmospheric phenomena go, lightning is extremely common. At any given second, 100 bolts of lightning are striking somewhere on the planet. Cloud-to-cloud strikes are five to 10 times more common. Lightning typically occurs during thunderstorms when the atmospheric charge between a storm cloud and the ground or a neighboring cloud becomes unbalanced. As precipitation is generated within the cloud, it builds up a negative charge on the underside.
This causes the ground below or a passing cloud to develop a positive charge in response. The imbalance of energy builds up until a bolt of lightning is released, either from cloud to ground or cloud to cloud, restoring the electrical balance of the atmosphere. Eventually, the storm will pass and the atmosphere's natural equilibrium will be restored. What scientists aren't yet sure of is what causes the spark that triggers the lightning bolt.
When a bolt of lightning is released, it is five times hotter than the sun. It's so hot that when it tears across the sky, it super-heats the surrounding air extremely quickly. The air is forced to expand, causing a sonic wave we call thunder. The thunder generated by a bolt of lightning can be heard as much as 25 miles away. It is not possible to have thunder without lightning.
Lightning typically travels from cloud to ground or cloud to cloud. The lighting you see during a typical summer thunderstorm is called cloud-to-ground. It travels from a storm cloud to the ground in a zigzag pattern at a rate of 200,000 miles per hour. That's way too fast for the human eye to see this jagged trajectory, called a stepped leader.
When the leading tip of the lightning bolt gets within 150 feet of an object on the ground (usually the tallest in the immediate vicinity, like a church steeple or a tree), a bolt of positive energy called a streamer surges upward at 60,000 miles per second. The resulting collision creates the blinding white flash we call lightning.
National Severe Storms Laboratory