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Why are cloud white?

Clouds are made of water vapours which are much larger than the air molecules. so Rayleigh's criterion does not apply

According to the Rayleigh's criterion, the intensity of scattered light is inversely proportional to the fourth power of the wavelength if the size of the scatterers are of the order of the wavelength of light. Air molecules have their size of this order.

However, as we have seen water vapours are much larger than air molecules and all colors of light are scattered equally. Thus all colors of light combine to make white light.

You may also see: Why is the sky blue?, Why is the sun reddish during sunrise and sunset and Why are rain clouds dark?