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A liquid drop is spherical in shape due to surface tension. But why does it appear as a vertical line under the free-fall due to gravity? (E.g. During a rain - falling raindrop) Is there a specified length for the line or does it vary with the size of drops?

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Your original question was better--- they appear long because they are moving fast, and so streak in your eye. –  Ron Maimon Aug 9 '12 at 8:32
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I'm not sure I exactly understand your question but maybe that can be interesting ems.psu.edu/~fraser/Bad/BadRain.html –  aberration Nov 2 '12 at 14:51
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up vote 6 down vote accepted

A drop that is free falling in vacuum is spherical. This is because free falling in a gravitational field is the same thing as being at rest with no gravitational field present: the gravitational field and the acceleration cancel each other out.

Rain drops falling to the earth can have various shapes depending on their size, although I am not aware that they can become elongated (can you provide a source?). These shapes are due to the air flowing past them, in a rather intuitive way: roughly, air flow causes the bottom to become flatter.

Edit: Regarding the appearance of raindrops (as opposed to their physical shape), consider taking a photograph of falling rain. The camera integrates the incoming light over the exposure time $t$, during which the drop travels a distance of $v t$, where $v$ is the velocity. If we are close to the ground this will be the terminal velocity, which is about $2_{m/s}$. If we use an exposure time of $t=1/60_s$ (say we are using a flash), the drop will trace a line of length $\sim 3_{cm}$. The apparent line on the photograph then has to account for distance from the drop, etc.

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They don't become flat, they become elongated, OP is asking why. –  Ron Maimon Aug 9 '12 at 3:53
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Their shape depends on their size. Small drops are spherical, larger ones look sort of like pancakes (this is what I mean by 'flat', perhaps this was not clear), yet larger ones look like parachutes. I don't know at which size (if any) they become elongated, but the shape is due to the flow of air and not gravity. I will try to clarify... –  Guy Gur-Ari Aug 9 '12 at 4:38
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I mistook a drawing for a photo! You are right--- raindrops are flat on the bottom. I assumed this would be unstable because the raindrop weight would be greater at the center. I found this paper: weather.about.com/od/cloudsandprecipitation/a/rainburgers.htm (although the wikipedia link you provide is fine). –  Ron Maimon Aug 9 '12 at 5:06
    
@CrazyBuddy then that's the question about optical illusions –  Yrogirg Aug 9 '12 at 7:53
    
@CrazyBuddy What confuses me at least is that you call them 'horizontal' lines -- I never saw a raindrop that looks like that. That is why I thought you meant this flattening. Do you mean vertical lines? –  Guy Gur-Ari Aug 9 '12 at 13:29
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