Every second, invisible particles from space, cosmic rays, strike Earth's atmosphere. Colliding with air molecules, they produce showers of secondary particles, among which muons are especially abundant. A muon is a heavy cousin of the electron that lives only a couple of microseconds but still manages to reach the ground. At the GRAPES-3 observatory in southern India, a giant telescope covering 560 square meters counts these muons around the clock. In a single day, it tallies about four billion particles — statistics so precise that hourly variations in the flux are visible at the level of hundredths of a percent.

It turns out muons are excellent thermometers. When the upper atmosphere (above 20 kilometers) warms, the air expands, and muons have to plow through more material. Some of them decay along the way, so the flux at ground level weakens. Conversely, a cold stratosphere contracts, allowing more muons to reach the surface. Over 22 years of observations — from 2001 to 2022—scientists compared the muon flux with temperature data from the MERRA-2 meteorological database. They found that a one-degree warming of the upper atmosphere reduces the muon flux by 0.22 percent.

The second factor is the interplanetary magnetic field, generated by the solar wind. The ACE and WIND satellites, positioned at the L1 Lagrange point between Earth and the Sun, continuously measure this field. It turns out that an increase of one nanotesla (roughly a hundred thousand times weaker than a refrigerator magnet) reduces the muon flux by 0.57 percent. It's as if a giant invisible shield alternately appeared and vanished in front of Earth, changing the number of particles that get through.

The practical benefit is clear: instead of expensive satellites for stratospheric and space weather monitoring, ground-based muon telescopes can be used. They operate continuously, are unaffected by clouds, and deliver data every few minutes. Thus particle physics becomes a weather service, but for weather at 30 kilometers altitude and millions of kilometers from Earth.