Fly the friendly skies—and make it rain. That's what researchers have found airplanes tend to do when they take off or descend through clouds of the altocumulus variety. The clouds, which range in altitude from about 1 to 6 kilometers, have been known to develop gaping holes or channels near airports, though no one knew why. Nor could anyone explain why their appearance immediately brought localized precipitation. But this month in the Bulletin of the American Meteorological Society, researchers report that when a jet or turboprop flies through clouds containing droplets of supercooled water, which can remain liquid even as low as -15°C, the droplets condense after passing over the aircraft's wings (or the blades of its propellers). The now-frozen droplets immediately fall to earth as rain or snow, and their absence creates the characteristic circular hole or tunnel effect in the clouds.
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