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Page 2
The Experiments of Phillipp Lenard
Our first understanding of raindrop shapes emerged from the experimental work of renown German physicist Philipp Lenard, a 1905 Nobel Laureate in physics for his work with cathode rays. Lenard began studying at raindrops in 1898, but did not publish the results of his extensive investigations on the shape, size and stability of raindrops until 1904. [For more on Lenard, click here.]
Much of Lenard's research interest focused on the behaviour of raindrops as they fell from clouds. Since he could not study the raindrops directly as they fell within and below clouds, he required some means to simulate their fall and the surrounding airflow. He accomplished this by constructing an innovative vertical wind tunnel in which he could vary the airflow to simulate atmospheric updrafts. Through adjustments to the airflow rate, he found he could briefly balance a drop in the air stream. This balancing act simulated the aerodynamic forces acting on a drop falling freely through a still air column. And a balancing act it was, for the turbulence levels in his wind tunnel were so great that drops could be held steady for only a few seconds. With the wind tunnel, Lenard could observe the actual shape a raindrop took while falling because a water drop takes on the same shape whether it is falling through still air or holding its position in an updraft. By suspending many drops of known diameters, Lenard determined that small drops, those less than about 2 mm (0.08 inches) in diameter, fell as spheres. Larger drops, however, were deformed while falling having a flattened bottom and rounded top. Lenard was therefore the first to report that falling raindrops were not teardrop-shaped but were spherical when small and "hamburger-bun shaped" when larger. Drops suspended in the airflow became unstable, however, at diameters greater than 5.5 mm (0.21 inches). They survived less than a few seconds before breaking apart in the airflow, torn asunder by the aerodynamic forces acting on the drop. This observation, combined with a lack of drops larger than this diameter in his raindrop-size field measurements, led Lenard to conclude that the maximum drop size possible in nature was around 5-6 mm. Despite the knowledge that raindrops are not tear-drop shaped, we continue -- yours truly included -- to depict raindrops in that old familiar shape. Perhaps the streamlined shape of the tear drop implies its falling motion more than the true raindrop shape does.
The copyright of the article Raindrop Shape: No More Tears - Page 2 in Meteorology is owned by . Permission to republish Raindrop Shape: No More Tears - Page 2 in print or online must be granted by the author in writing.
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