How Hot Is Lightning?

(Inside Science) — Lightning is one of the most destructive forces in nature. But for all the folklore and legends amassed over human history on lightning, we know surprisingly little about the inner workings of this powerful phenomenon, including something as simple as how the current that flows through a thunder-inducing flash is related to the temperature of the strike.

"The basic physics of lightning, such as lightning initiation and lightning propagation, is not fully understood at this point," said Robert Moore, a lightning researcher from University of Florida in Gainesville.

"We know the basics, but not the details. So when anybody makes headway, it is major news."

Lightning causes more than $5 billion in damages every year in the U.S., as well as more fatalities than hurricanes.

"A direct hit from a lightning strike can melt a power cable or start a forest fire, where the amount of heat from the lightning plays a major role," said Xiangchao Li, a scientist from China who specializes in lightning research. Li and his team discovered a mathematical relationship between the current intensity and the temperature inside lightning. Their result was published last month in the journal Scientific Reports.

Although there are approximately 100,000 lightning strikes on Earth every single day, the randomness of the occurrences makes it difficult for scientists to study them in an effective or systematic way. So until Thor, the Norse god of lightning as well as other meteorological events, joins a lightning research team, scientists are left to their own devices.

Luckily such a device does exist. Known as an impulse current generator system, the device can create artificial lightning with currents up to tens of thousands of amperes. For perspective, a household or automotive fuse is usually rated well below a hundred amperes, and an electric current of just a few amperes can easily kill you. A natural lightning strike typically carries around 20-30,000 amperes of current. Certainly there are other factors such as size and setting of natural lightning that cannot be replicated in a laboratory, but just in terms of sheer current output, the lightning generated by the device can really give Thor a run for his money.

By using their artificial lightning system, Li and his team were able to dial up lightning strikes at will, with currents between 5,000 to 50,000 amperes. This resulted in artificial lightning strikes with temperatures as high as 17,000 F, twice as hot as the surface of the Sun.

This creates a new problem — at such high temperatures, a normal thermometer would explode. And even if it didn't, it wouldn't react quickly enough to register the temperature of the lightning strike. Fortunately, there is "light" in "lightning." Li and his team were able to record the lightning's temperature within a millisecond by measuring the intensity of the light at various wavelengths.

After striking lightning at the same place over and over again, they concluded that the relationship between the current and temperature of lightning is a highly logarithmic one, meaning that the temperature difference between lightning strikes with 1,000 and 10,000 amperes is similar to those with 10,000 and 100,000 amperes. This result provides solid evidence for previous theoretical predictions that lacked the support of data.

"The next step would be to compare with measurements from rocket triggered lightning, or natural lightning, which can be done throughout the U.S. or China," Moore suggested.

That's right, rocket-triggered lightning. Essentially a glorified version of Benjamin Franklin's wired kite, scientists today have ways to siphon natural lightning from the sky by launching an electrically grounded rocket, as shown in the video below.

With a better understanding of the physics of lightning, scientists can help engineers to improve current protocols and infrastructures to better deal with lightning -- from weather warning systems to the design of power grids. Perhaps we can one day limit the power of Thor to only smiting Loki on the silver screen.

Editor's note: Quotes from Xiangchao Li are translated from an original interview conducted in Chinese.

This article is provided by Inside Science News Service, which is supported by the American Institute of Physics.