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When we think about raindrops, we often have a fixed mental image of how they look. Our biggest misconception about raindrops concerns their shape as they fall from the heavens.
Imagine a raindrop in your mind, or ask a friend to do so. Odds are you/they will form a mental picture of a tear-drop-shaped water mass with a rounded front with a pointed tail. Literature, poetry and song are filled with allusions to raindrops as heaven/sky tears, and those mental images have clouded our minds. The metaphor likely began when someone noticed that raindrops hitting a hard surface, such as window glass, roll off like tears flowing down the cheek. In truth, raindrops are spherical in shape when they begin to fall. Then, unless they are very small, they take on shapes with flattened bases and rounded tops, looking more like falling hamburger buns than teardrops, as they fall. The distortion from the spherical drop is caused by the air resistance against the drop as it descends, which flattens the lower drop surface. This aerodynamic drag force can further deform large drops into sagging dumbbell shapes, causing the biggest ones to eventually split into two or more smaller drops. In actual fact, even the hamburger-bun shape for raindrops, which is based on the observations of single drops steadily falling through a nearly non-turbulent airstream, is idealized, particularly for heavy rain events. If we could isolate single drops in a rainstorm and follow each from its formation until its final splashdown, we would see, not a tear drop, nor a steady spherical or bun-shaped mass, but an ever-changing, quasi-spherical shape throughout its lifetime. When rain falls, its multitude of drops come in many different sizes. Each drop falls at a slightly different speed from those smaller or larger as the aerodynamic drag force (which is dependent on the drop diameter) balances the pull of gravity. Indeed, the smallest drops may not fall at all, being suspended or perhaps forced upward by ascending currents of air until they grow large enough to overcome the updraft. As a consequence of the differing drop speeds, there are many collisions among raindrops. Some collisions cause distortions in the drops' shapes as they bounce off one another. Other collisions cause drops to coalesce, forming a larger drop in the process. And some collisions cause drops to break apart into smaller drops. As raindrops fall to earth, each may grow in size by collecting other drops during descent, or perhaps splitting apart due to collisions or growth to an unstable size. As the drops change their size (their volume), they jiggle, joggle and wobble through a variety of spherical distortions.
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