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Friday, 29 November 2013

The Shape of a Raindrop

     Despite weather channel depictions and after years of drawing raindrops with a pointed top and rounded bottom, I was surprised to learn that raindrops are not shaped like teardrops.
     Air resistance is not the cause of the typically-drawn shape that appears as raindrops streak across windows or on peoples' face when we dry; that is caused by surface tension of the liquid water on the surface.
     As they fall through the air, raindrops are actually spherical in shape. Smaller raindrops have less air resistance so they are more spherical than larger ones. The larger the raindrop, the more air resistance there is, and greater air resistance actually just flattens the bottom; a pointed tail/trail is actually not formed by air resistance. When a raindrop gets large enough (usually more than 4mm), it will actually split in half and drastically decrease in surface tension, possibly splitting so small that the water just rejoins as moisture the atmosphere.

Sources
Baird, Christopher S.. What Makes Raindrops Tear-Shaped? December 17, 2012 Science Questions with Surprising Answers. http://sciencequestionswithchris.wordpress.com/2012/12/17/what-makes-rain-drops-tear-shaped/

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