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Why is the sky black when it rains?

Why is the sky black when it rains?
Craig Hill from Tyne and Wear (Age 5-14)

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One Response

  1. Hi Craig,

    Generally Clouds look white when you see them from above because the droplets in them are very small and they are just the right size to reflect and scatter the visible light in lots of directions. Also the scatterng is the same for all wavelengths. When the air has a lot of moisture in it, there is enough condensation to form larger drops and eventually these drops get big enough allow gravity to pull them down to earth as rain. When the drops get this big, they do not scatter the light as well as the very small droplets. The light is more likely go into the droplet and either get stuck in by reflections and absorption or alternatively it can only get back out at specific angles(and make a rainbow). Also, for some angles the light is 100% trapped in the drop by total internal reflection.

    If we look up at the cloud at this point, we will see less light coming from the cloud because it is trapped in the droplets and the cloud will look very dark. Also the more large rain drops there are, the darker the cloud will look. So it is a good predictor that you might get wet.

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