You've probably seen hundreds of lightning flashes. Scientists are still working to discover some of its elusive secrets, but here's the basic story:
Lightning is an atmospheric discharge of electrical energy. When thunderstorm clouds develop, ice crystals (or other particles of precipitation, such as rain or snow) move around in the clouds because of air currents, and the particles collide with one another. Some particles lose electrons during the collisions, and a negative charge builds up in the lower portion of the storm cloud. Air initially acts as an insulator between areas of positive and negative charges, but when the charge difference becomes sufficiently large, lightning is the result.
The most common type of cloud-to-ground lightning is called a negative flash. This occurs when a negative charge in the lower portion of a cloud results in the building up of a positive charge in the ground beneath the cloud. When the opposing charges are strong enough, a branched pathway of negative charge descends from the cloud leaving an ionized trail in its wake. This is called a leader. As the leader nears the ground, a streamer of positive current stretches up toward it. When the leader and streamer connect, electrical current surges through the ionized path. This is called the return stroke. It releases massive amounts of electrical energy as it moves back up through the leader at a speed of about 200 million miles per hour. The combined duration of the leader's descent to the ground and the return stroke is just a small fraction of a second.
A positive flash occurs in a similar fashion, but it originates from the upper portions of a cloud. Normally, the upper area of a cloud is shielded from the ground by the middle and lower parts of the cloud, but wind conditions can sometimes cause a thunderstorm's cloud to be tipped or spread out away from the main portion of the storm. Although positive flashes occur more rarely than negative flashes, they can pose a bigger danger because they can strike with little warning at large distances from the main area of storm activity.
Another type of lightning is called cloud-to-cloud lightning. It can occur between portions of a cloud that have different electrical charges (called intracloud lightning) or between different clouds (intercloud lightning). Cloud-to-cloud lightning is more common than cloud-to-ground lightning, and it is less dangerous to people and ground-based structures.
Lightning causes the air in its channel to become hot—very hot—three to five times hotter than the surface of the sun. This causes the air to expand and contract quickly. The shock wave that results from this rapid temperature change creates thunder. The flash of lightning and the thunder are created at essentially the same time, but because light travels through the atmosphere faster than sound, you see the flash before you hear the thunder (although you might not see the flash at all if it is hidden by clouds or obscured by your surroundings).
You can estimate how far away lightning is by counting the seconds between the flash and the thunder. Every five seconds is the equivalent of one mile, so if you count to five, the lightning flash is one mile away. When you hear the thunder, the sound that reaches you first is from the portion of the affected air closest to you. The loudest portion of the boom is associated with the main point at which the lightning reached the ground. Rumbling can persist as sound waves continue to reach you from more distant points along the lightning's pathway.
This article, including a list of links for more information, originally appeared in the July 2015 issue of Observations: A Pier Press® Newsletter. You can access the archived issue here, and if you're not already a subscriber, click here to sign up. It's free.