From space, an invisible rain continuously falls upon Earth — a stream of muon particles born from the collision of cosmic rays with air. These 'drops' penetrate the atmosphere and reach the surface. The denser the air, the more muons are absorbed along the way — just as thick foliage catches drops of ordinary rain. By measuring how many make it to the ground, scientists can gauge the density of the atmosphere above us.
Researchers tested this idea on the tropical cyclone Freddy. They added data from a hypothetical muon detector — a compact device that can actually be built — into a computer model. Even such a modest room-sized sensor allowed for more accurate calculations of air density and the cyclone's development. The secret is that the muon method provides a three-dimensional picture of the entire atmospheric thickness, not just point measurements of pressure at the ground like traditional weather stations.
Most surprisingly, one such detector can replace an entire network of weather balloons and ground stations, because it 'feels' the air density at all altitudes simultaneously.
A muon is a heavy version of the electron, living for two millionths of a second. But thanks to time dilation from relativity, it manages to reach Earth, though it should have decayed almost instantly.