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The pitter-patter of raindrops on your umbrella is caused by raindrops of all different sizes, and now physicists have a new explanation for how those raindrops form. A pair of researchers used a high-speed camera (video below the jump) to watch a single drop of water fall and change shape over the course of six-hundredths of a second, and found that the shattering of single raindrops after they leave clouds is enough to explain the wide variety of drop sizes [Science News].

Previously, the leading theory to explain the diversity of raindrops had been that raindrops grow as they gently bump into each other and coalesce. Meanwhile, more forceful collisions break other drops apart into a scattering of smaller droplets. All this action would explain the wide distribution of shapes and sizes [ScienceNOW Daily News]. But lead researcher Emmanuel Villermaux says he questioned that theory, with its supposition of frequent collisions. Real raindrops are so sparse, he said, that it is likely a drop would “fall on its own and never see its neighbours” [BBC News].

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Who knew this spring’s soggy weather fell under the umbrella of physics research? Scientists found that when raindrops fall faster than physics predicts, the drops have actually broken into smaller droplets, according to a study in the journal Geophysical Research Letters. And because weather services gauge rainfall based on the velocity at which droplets fall–conventional wisdom holds that large drops should hit the ground at a higher speed than do smaller droplets–these results could improve the way we predict weather.

All falling objects have a so-called terminal velocity, a speed they can’t surpass due to air resistance. Therefore, larger drops generally should fall faster because their heftier size helps them power through air resistance more easily than little drops. (In the extreme case, think of fog: water droplets so small they don’t fall at all.) But data showing small drops sometimes impact the ground at the same speed as larger ones showed this conventional wisdom was wrong, and has puzzled scientists for years. To solve the mystery, the researchers collected a shower of data using optical equipment over a period of several years. The team clocked about 64,000 raindrops falling in Mexico City. The researchers measured their sizes and velocities only in extremely calm conditions, so the wind that often accompanies rain could not skew the data. They found that some drops plummeted faster than the so-called terminal velocity for their size [ScienceNOW Daily News].

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