The WWLLN currently comprises more than 60 stations, distributed unevenly around the world, which detect radio waves produced by lightning strikes. With this method, the network can monitor clusters of lightning activity from thousands of kilometres away.

"The WWLLN stations register the GPS time of every detected lightning discharge," explained Keren Mezuman from Columbia University and the NASA Goddard Institute for Space Studies, US. "Using the difference in the time of arrival of the signal at five stations, the location for individual lightning strikes is obtained anywhere on the globe."

Analysing the last seven years of data from the WWLLN, Mezuman and her colleagues have shown that lightning activity peaks at around 19 Coordinated Universal Time (UTC). Meanwhile, atmospheric fireworks fall to a minimum around 3 UTC every day. These times match almost perfectly with the peaks and troughs of the Carnegie Curve – the global variation in Earth's vertical electrical field, which was first measured in the 1920s aboard the geophysical survey vessel of the Carnegie Institution of Washington.

Zooming in on specific regions, the data show that thunderstorm activity peaks across Africa around 14 UTC, and around 19 UTC across South America. Previous work has shown that Africa dominates when it comes to lightning discharges, but Mezuman and her colleagues found that it is South American thunderstorms, not African ones, that dominate global thunderstorm activity. The scientists speculate that although the African storms contain more frequent flashes of lightning, they might be weaker flashes and smaller storms, which would imply they may not contribute as much electrical current to the global electrical circuit as the South American storms.

When it comes to spatial distribution, Mezuman and the team discovered that around half of all thunderstorm cells occur over the oceans. Previous measurements, in contrast, have suggested that 90% of lightning occurs over land. The mismatch may be due to the WWLLN's tendency to detect intense lightning strikes, more prevalent over the oceans. "Our study shows that when we consider thunderstorm cells, around 50% of them are over the oceans," said Mezuman. "This implies that thunderstorms themselves are much more evenly divided between land and ocean, while the lightning activity in the land storms is much higher, giving the 90:10 ratio in lightning amounts." The findings are published in Environmental Research Letters (ERL).

As the coverage of the WWLLN grows, it could become a useful forecasting tool. "This new real-time information can be used to now-cast storms in remote areas," said Colin Price from Tel Aviv University, Israel, a co-author on the paper. "By tracking the clusters we can predict where they will be in the coming hours, and warn the public." Ultimately, the researchers also hope to be able to track the development of tropical storms and hurricanes, by monitoring the thunderstorm clusters brewing over the oceans.

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