Scottish physicist Charles C. T. R. Wilson became the first to observe the motion of individual alpha or beta particles in a cloud chamber he had specially invented. Fifteen years earlier, Wilson had taken meteorological measurements for two weeks from the summit of Mount Nevis, the highest mountain in the British Isles, to study clouds and their formation. Upon returning to Cambridge, he performed several confirmatory experiments. He had thus understood that when air expands due to the drop in pressure at altitude, it becomes supersaturated. Under the right conditions, the humidity in the air then condenses around small dust particles or ionized particles.
