What Happens During a Lightning Storm?

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. 

Causes

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.

Dangers and Safety Tips

In the United States, lightning occurs most often in July, typically in the afternoon or evening. Florida and Texas have the most strikes per state, and the Southeast is the region of the country most prone to lightning. People can be struck directly or indirectly. Although the vast majority of people struck by lightning survive, about 2,000 are killed worldwide every year, usually due to cardiac arrest. Those who survive a strike may be left with damage to their cardiac or neurological systems, lesions, or burns. 

When a thunderstorm occurs, you can do some simple things to protect yourself from lightning strikes, whether you're indoors or outside. The National Weather Service recommends the following precautions:

• If you're outside, seek immediate shelter. Houses and other substantial structures with indoor electricity and plumbing, which are grounded, are your best option. Vehicles with solid tops (not convertibles) are also grounded and safe.
• If you are caught outdoors, move to the lowest ground possible. Do not seek shelter beneath trees or other tall objects.
• Avoid plumbing or running water. Metal pipes for water and sewage are not only excellent conductors of electricity, but the water they carry can be laden with impurities that also help conduct electricity.
• Don't use landline phones with cords or desktop computers. Electricity can also be transmitted through the wiring of your home. Cordless and mobile phones are safe to use. 
• Stay away from windows and doors. Lightning is a gorgeous sight, especially when arcing across a night sky. But it has been known to strike people after passing through glass or unsealed cracks along doors and windowpanes.

Sources

• Centers for Disease Control and Prevention staff. "Frequently Asked Questions (FAQ) about Lightning Strikes." cdc.gov.
• Moskvitch, Katia. "We Don't Actually Know What Triggers Lightning Strikes." Slate.com, 18 August 2013.
• National Geographic staff. "Lightning." NationalGeographic.com.
• National Severe Storms Laboratory staff. " Severe Weather 101: Lightning." nssl.noaa.gov.