Some 3 million lightning flashes occur on Earth every day, which amounts to 30 flashes per second, and while many of these pass from cloud to cloud, a significant number reach the ground. In the United States, approximately 20 million ground flashes occur annually, causing an average of 54 deaths and many more injuries. Being struck by lightning may be statistically improbable, but it's definitely possible, and you need to take steps to protect yourself.
Lightning and Thunder
Lightning is an atmospheric discharge of static electricity that builds up in clouds as a result of the upward and downward movement of water and ice droplets. Friction generated by these droplets creates a negative charge in the lower portion of a cloud and a positive charge in the upper portion. When the charge is large enough to overcome the insulating effect of the atmosphere, an arc of electricity jumps to another cloud or to the ground. The arc superheats the air, causing it to suddenly expand and produce the crack of thunder. The thunder turns to a rumble as the air continues to vibrate.
Two Types of Ground Lightning
The most common type of ground lightning issues from the lower portions of clouds. It attracts positive charge from the Earth, which accumulates at a particular point and creates streamers that reach up to meet the stepped leader coming from the cloud. A second -- more dangerous -- type of lightning issues from the tops of clouds. This positive lightning, which constitutes 5 percent of all lightning strikes, has a larger charge than ordinary lightning, giving it the power to travel long distances -- as much as 16 kilometers (10 miles) from the center of the cloud.
Avoid Tall Objects
Because air is an effective electrical insulator, an electrical charge rising from the Earth minimizes the distance it has to travel through air on its way to meet the stepped leaders coming from the clouds. It therefore climbs through the tallest object in the vicinity of the cloud, which may be a tree or a tall building. If you're caught outside during a storm, you should stay clear of tall objects like trees and buildings because these direct electricity to the ground, and it can travel through your shoes. If there is nothing nearby, you should crouch down to avoid making yourself the tallest object. Don't lie down, because lightning travels through the ground as much as 30 meters (100 feet) from where it strikes.
General Safety Rules
When you see a lightning flash, count the number of seconds until you hear thunder, and seek shelter if the count is less than 30. Appropriate shelter is any closed building with a roof, four walls and an insulating floor or a vehicle with a roof -- avoid open garages, patios and convertible automobiles. When inside, avoid using water or landline telephones because lightning travels through plumbing and telephone wires. You should stay indoors for 30 minutes after you hear the last clap of thunder -- most casualties happen after a storm has passed and people go outside prematurely. After 30 minutes, you can be sure the storm is too far away to strike you with a bolt of positive lightning.