Every day, about 40,000 thunderstorms flash across the planet. In that moment, the atmosphere resembles an orchestra without sheet music: warm air rises like a bow stroke, winds aloft carry the melody, and lightning punctuates the finale. But who conducts this orchestra? Scientists set out to understand why thunderstorm hotspots shift in some years and stall in others, and they built a map of 'thunderstorm climate' spanning 45 years.

To hear the music of storms, the researchers didn't chase every cloud. Instead, they used ERA5 reanalysis data, which is like a history of the atmosphere reconstructed from fragmentary records since 1979. For each city-sized grid point on the planet, they computed two numbers: how much energy is available to lift air upward (like a fuel reserve) and how strongly wind changes with height (like string tension). Multiplying them gave a thunderstorm environment index. The threshold for a 'storm day' was tuned so that the count matched actual lightning flashes detected by sensors over the past 12 years.

Then the conductors took the stage. El Niño, the warm phase of the Pacific oscillation, proved to be the lead: about a quarter of the planet dances to its baton, and in tropical oceans that share reaches 52%. The Indian Ocean Dipole chimes in during autumn, but its rhythm often blends with El Niño. Polar oscillations play their part in high latitudes, where tropical motifs fall silent.

Over 45 years, 15% of grid points saw more thunderstorm environments, while 8% saw fewer. But these changes are still quieter than natural noise: from year to year, the number of storm days fluctuates by 28 on its own. Climate models predict that for each degree of warming, about 30 additional thunderstorm days per year will occur, mainly because moist air is more eager to rise.

Previously, it was unclear how strongly global modes influence thunderstorms in different regions and whether a warming trend could be seen in the data. Now we have a quantitative map: El Niño explains up to 52% of variability in tropical oceans, and the warming signal is comparable to natural spread. By comparing a unified index, calibrated against lightning, with four climate indices and a model ensemble, the team managed to separate the conductor from the orchestra.

This map will be useful for seasonal forecasts: knowing the phase of El Niño, one can warn farmers, energy providers, and emergency services in advance about likely thunderstorm outbreaks. The storm orchestra is still full of improvisation, but at least now we can see who holds the baton.