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Collecting rain water

  1. Sep 28, 2011 #1
    suppose rain water is being collected in bucket. does the rate at which it fill depend on wind?
  2. jcsd
  3. Sep 28, 2011 #2
    Do you get wet faster on a very windy day??

    Freefalling rain has some modest terminal velocity...that can easily be exceeded on a windy day delivering more rain per unit area.
  4. Sep 28, 2011 #3


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    Say your raindrops were separated by 1 cm as they fell vertically. Then say the wind speed matched the vertical velocity of the raindrops. The raindrops would fall at a 45 degree angle.

    If you're measuring the separation between raindrops perpendicular to their path, the path of the raindrops is now 1cm * cos(45). In other words, their paths are closer together.

    Because the raindrops are falling at an angle, the cross-sectional diameter of the bucket decreases. Since the raindrops are falling at a 45 degree angle, the cross sectional diameter would be the diameter * cos(45).

    The actual angle used (i.e. wind speed) won't matter, because you'll be multiplying by the cosine of the angle for both the separation between raindrops and the cross sectional diameter of the bucket.

    In other words, both the separation and the cross sectional diameter are decreasing at exactly the same rate. As a result, you can completely ignore the effect of the wind and just look at the separation that would exist if they were falling vertically.

    Now if the raindrops were falling straight down and you instead just tilted the bucket, then the bucket would fill at a slower rate because only the angle of the bucket relative to the raindrops was being changed - the raindrops would still be separated by a constant amount.

    There is a limit to that, however. Conceivably, the wind could be blowing so hard the raindrops are moving horizontally or even rising (which is how hail forms, except at high altitudes where the temperature is colder). In that case, the bucket wouldn't fill at all.

    And, to answer Naty's question - yes, you do get wetter on a windy day, since the surface area of the front of you is much larger than the surface area of the top of you. Well, at least unless you have some severe glandular problems and assume the shape of a sphere (see the "transversing in rain" thread).
  5. Oct 16, 2011 #4
    Hold on,
    Suppose the wind was blowing horizontally, but the rain continued to come into the airstream. Doesn't this imply that the airstream could support an infinite quantity of water?
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