Lightning Detection 101

How do they know if a bolt struck the mine in West Virginia?

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According to a local newspaper report, a series of lightning strikes may have caused this week’s explosion in a West Virginia mine. Vaisala, a company that keeps track of thunderstorms for the government, says a pair of lightning bolts hit near the mine around the time of the explosion. How do you detect lightning?

With a nationwide network of electromagnetic sensors. Vaisala’s U.S. National Lightning Detection Network picks up almost every lightning strike in the country and transmits its location—with an accuracy of about 500 meters—to a central processing facility via satellite. The company’s computers store detailed lightning-strike data going back to at least 1994. In that time, there’s been an average of 25 million flashes of cloud-to-ground lightning per year. (Each “flash” you see on the horizon comprises several individual “strokes” of lightning.)

Vaisala’s network sensors look like oversized Q-tips that are half-buried in the ground. Each one, which is only a few feet high, contains antennas to pick up both magnetic and electric fields. By combining the two signals, the sensor can determine the direction and intensity of a lightning strike. As long as two or more sensors get a reading, the network can home in on the area where the lightning hit.

The Bureau of Land Management set up a partial lightning detection network in the 1970s. Other local networks sprang up over the years; by 1989, they were capable of generating a lightning map for the whole nation. Vaisala, a private company, now operates a national network and sells lightning-detection information to anyway who’s willing to pay. At the Vaisala Web site, you can get a free map of recent lightning strikes in the United States, or search for lightning strikes on a particular day at a particular place for $90 and up. For a lot more money, you can get a long-term subscription with more complete data. (One of Vaisala’s few competitors, the United States Precision Lightning Network, says its own system can pinpoint individual strokes of lightning to within 250 meters.)

If you want local lightning information but don’t want to pay for Vaisala’s service, you can buy a detector and set it up yourself. These can range from hand-held devices that cost about $100 to more complicated systems that run $10,000 or more. Some combine electromagnetic sensors with optical monitors that check the sky for flashes of light. Satellites can also detect lightning by looking for the flashes.

Bonus Explainer: Is there more lightning now than there used to be? It’s not clear. The number of flashes registered by the network varies by as much as 50 percent from year to year. Without decades of data, it’s very difficult to discern long-term trends. Right now, lightning strikes the Earth about 100 times per second, but one lightning expert told the New York Times in 2001 that global warming could cause that number to double or triple by the middle of the 21st century.

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Explainer thanks Richard Kithil, Jr., of the National Lightning Safety Institute, Nicholas Demetriades of Vaisala Inc., and Vladimir Rakov of the University of Florida.