Electrifying Weather: Lightning Bolts Out Of The Blue

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Lightning strikes the ground somewhere on the earth 140 million times per year. Graphic courtesy of NOAA.

Lightning strikes the ground somewhere on the earth 140 million times per year. Graphic courtesy of NOAA.

Sharks kill approximately 15 people per year worldwide. In the United States alone, 51 people are killed by lightning each year, while a fatality from sharks’ teeth is rarer than (sorry) hens’ teeth.

Sharks get the publicity, but lightning is much more dangerous.

A lightning discharge occurs somewhere around the globe about 44 times per second; four million times per day; approximately 1.4 billion times per year. 90% of the strokes are in a cloud or from one cloud to another. That leaves 140 million bolts of lightning that hit the ground.

A stroke of lightning generates a billion volts of electricity. That’s over a quintillion (ten to the 18th power) volts per year. Where does all that voltage come from?

When you think about it, there is a virtually unlimited supply of electricity in the universe: There are about 10 to the 80th power (ten followed by 80 zeroes) atoms in the universe, most of them hydrogen, so the number of electrons is a little more than that. There’s plenty of electricity available if you separate out the electrons.

How Does Lightning Form?

Lightning is an electrical discharge that results from an imbalance of charge within a cloud. The process by which this separation of charge comes about is not simple, nor even completely understood by meteorologists.

The process involves interactions among water droplets and ice crystals. Droplets and crystals become separated by size, as the larger ones fall and the lighter ones are supported by updrafts. As the drops and crystals rub past each other, a charge builds up similar to the one you get by rubbing your feet on a rug.

The end result is a negative electrical charge at the bottom of a cloud and a positive charge at the top. When the imbalance of the distribution of electrical charge within a cloud gets large enough, something has to give.

How Is The Electrical Imbalance Relieved?

The imbalance is relieved by a gigantic surge of electrons from the negatively charged part of the cloud to somewhere where there’s a positive charge.

There are three places the electrical discharge can go: A lightning bolt can travel from a negatively charged part of one cloud to the positively charged part of the same cloud; from the negatively charged part of one cloud to the positively charged part of another cloud; or from the negatively charged part of a cloud to the positively charged ground. These are called respectively in-cloud, cloud-to-cloud, and cloud-to-ground lightning. We are most concerned with the last one, because it is the one that kills most people, though lightning strikes in and between clouds can sometimes strike aircraft.

The Path Of A Lightning Bolt

The air is a good insulator, and a cloud with a lot of negative charge at its base has to probe to find the best route to the positively charged ground. That is why the path of a lightning strike is crooked. The electrons seek the easiest route, rather than the shortest, the way an explorer takes the most convenient trail through a wilderness, turning this way and that depending on the growth of vegetation and contour of the land. The lightning bolt wiggles and waggles its way to the ground, following the path of least resistance. Other things being equal, the lightning will strike the nearest object, but lightning has been known to strike the ground as far as twenty miles from the cloud.

Where Does Lightning Occur?

Lightning strikes are most frequent near the equator, but temperate land masses receive their fair share. Graphic courtesy of Metoffice.

Lightning strikes are most frequent near the equator, but temperate land masses receive their fair share. Graphic courtesy of Metoffice.

Luckily lightning strikes do not occur randomly; we know where lightning will strike — near a thunderstorm. Most of these thunderstorms fire up in the tropics, but outbreaks of violent weather are common in the central and eastern US., as well as temperate parts of every continent.

The most electric place in the world is hard to pin down — it depends on definitions. For a limited period of time, the start of monsoon season, the Himalayan foothills of eastern India have nearly constant lightning. But on a yearly basis, the prize for most lightning goes to the area where the Catatumbo River meets Lake Maracaibo in Venezuela. Tapping the moisture sources of the lake and the Caribbean Sea, thunderstorms form daily. Mountains on three sides trap the storms, which normally begin in late afternoon and last into the wee hours, averaging 28 lightning flashes per minute.

Why Do We Call It A Thunderstorm?

Lightning and thunder are two manifestations of an electrical energy release. In technical terms, the potential energy of separated electrical charges is turned into energy of light and sound. The fact that we focus on the sound when naming the event is purely a matter of choice. Maybe the boom seems scarier than the flash of light. But it’s the discharge of electricity that kills, so call it what you want: Electrical Storm is probably most descriptive.

Sound and light travel at different speeds, so the thunder and lightning reach an observer on the ground at different times. The light travels at, well, the speed of light, or 670 million miles per hour (200,000 miles per second). At any distances from which a lightning bolt can be seen, the flash reaches a person on the ground virtually instantaneously.

The comparison between the speeds of thunder and lightning is like that of snail mail and email. Sound travels through the atmosphere at about 760 miles per hour (0.2 miles per second). At that rate it takes thunder about five seconds to go a mile. That’s the basis for the common rule that you can tell how far away a thunderstorm is in miles by counting the seconds between the flash and the thunder and dividing by five.

Since sound is attenuated by air (loses strength as it moves through), thunder is barely audible twenty miles from the source, while lightning can be seen much farther away. Lightning that is far enough away to be seen but not heard is sometimes called ‘heat lightning,’ but the name has little meaning with regard to temperature. All lightning is hot —  up to 50,000 degrees Fahrenheit. The surface of the sun, by contrast, is about ten thousand degrees.

Stay Safe

If you can hear the thunder, you can be struck by lightning. Inside a car or structure is safe, as lightning hitting the building or car will be directed around you to the ground. In the open, it is better to seek a low spot than to get under a tree. Lightning striking the tree can be conducted through the ground to you.

To be safe, stay away from both sharks and lightning.

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