One significant source of cloud heating is a ubiquitous flux of particles whose existence has been known for almost a century, but whose origin is only now becoming clear. By 1900, physicists were routinely using ionization chambers to measure minute amounts of emission from radioactive elements. Even when no substance was introduced, the chambers recorded the presence of some ionizing agent. Using such a device on board a balloon, V. Hess found, in 1912, that the flux increased with altitude and had no diurnal variation, and therefore was of extraterrestrial and extrasolar origin. It took several more decades to unravel the detailed nature of this bombarding stream.
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