IEEE University of Lahore


What Is ‘Hot Lightning’? Satellites Reveal Which Strikes Are Most Likely to Start Wildfires

Before the end of this year, Vaisala will debut a beta product that will make this valuable measurement available to clients for the first time. The product (which is now running but is not yet commercially available) combines data from its terrestrial U.S. and global lightning detection networks with new information from a pair of optical sensors, known as Geostationary Lightning Mappers (GLMs). The sensors are currently orbiting Earth aboard two weather satellites that belong to the U.S. National Oceanic and Atmospheric Administration (NOAA).  

“For the first time ever, maintenance and fire crews will be able to use an operational lightning data feed to target cloud-to-ground lightning events most likely to cause asset damage due to heating or start a wildfire,” says Ryan Said, research scientist and systems engineer with Vaisala.

The company’s goal is to use all of this data to detect the presence of a single phenomenon: something called a continuing current, which is thought to occur in about 11 percent of lightning strikes. Lightning that harbors a continuing current is more likely to start fires and damage homes or equipment.

To understand why, it’s helpful to remember that lightning strikes begin with an imbalance in electrostatic charge between storm clouds and the ground. For reasons that are still not entirely clear to scientists, negatively-charged particles often build up at the bottom of storm clouds, while positively-charged ones gather at the top of clouds and on the surface of the ground below.

This distribution creates an electric field that can mess with the structure of particles suspended in the air around it. The field essentially teases nearby particles apart to produce positive ions and a roving band of electrons. Together, these particles form a kind of ion-electron stew called a plasma, which happens to be an excellent conductor.  

Gradually, electrons from this stew begin to move toward the ground in roughly 50-meter segments that are likely steered in part by conductive pockets of air created by dust or other particles. At the last moment, another flow of positively-charged particles rises up from the ground, and the two flows meet in mid-air.

This interaction instantly establishes a channel connecting the clouds and the ground that many other electrons can follow. A lightning strike, then, is the flow of a great many electrons through this channel. As many as a billion trillion electrons can move through it in less than a millisecond, according to one source (note: lightning can also form in other ways not captured by this description).

Typically, all of this is over in (literally) a flash. But some strikes sustain a current for up to a thousand times longer than average—the aptly-named continuing current. During these strikes, electrons flow for tens of milliseconds longer than in flashes that do not have a continuing current.

Granted, a continuing current is not as powerful as the flash itself. While a flash might have a peak current of 20,000 amps (averaged from the multiple composite strokes that make up a single flash), a continuing current measures between 100 to 1,000 amps.

But a continuing current lasts much longer—which makes it a highly effective fire starter. “If you have a hot poker and quickly touch an object, it might not do much damage,” explains Said. “But if you hold it there for awhile, it can heat up.”

Such “hot lightning,” as it’s called, can be spotted by the Geostationary Lightning Mappers, which detect rapid changes in brightness in the 777.4-nanometer (near infrared) band associated with lightning.

Scott Rudlosky, a physical scientist at NOAA, says certain characteristics (such as brightness and duration) can be used to recognize flashes that harbor a continuing current within the many flashes the lightning mappers record. “There is also an instrument artifact termed rebound that appears to coincide with long continuing current flashes, so this may prove to be a feature rather than a flaw,” he adds.

But the spatial resolution of the satellite data is too low to provide actionable information on individual flashes. That’s why Vaisala researchers plan to combine it with the company’s U.S. and global lightning detection networks to pinpoint exactly which flashes pose the greatest threat. Those networks are made up of dozens of magnetic field sensors that measure electromagnetic radiation emitted from lightning strikes.

With those sensors, Vaisala’s global lightning detection network can pinpoint the location of any cloud-to-ground lightning strike in the world to within 2 kilometers. For its U.S. network, that accuracy improves to within 200 meters.

When it comes to identifying a continuous current, “the combination of ground- and space-based observations is key,” says Rudlosky, “so Vaisala is in the best position to merge these observations.”

Said, the research scientist at Vaisala, says that once the beta product is live, the company expects that it should be able to identify flashes with continuing current about one minute after they occur. This capability would apply to strikes anywhere within the coverage area of the two GOES-R weather satellites that are carrying the lightning mappers, which includes most of North and South America.

Those two satellites will soon be joined in orbit by two more, each with its own lightning mapper. The set will provide coverage through 2036. “Our overarching goal is to ensure that the GLM provides the most value possible, so it is very encouraging to see private companies working with NOAA to exploit this new technology,” says Rudlosky.

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