Why are clouds different shades of gray?

We posed this question to Peter Braesicke from the University of Cambridge... Peter - So basically, the colour of the cloud is determined by the size of the droplets in it. As we experience when we have rain, there are different sizes of raindrops around, so different sizes of little particles in the clouds, droplets in the clouds. Depending on the size, the scattering changes, so the colour you perceive the cloud as changes and so you see it in different shades of grey. Ben - And it always feels instinctively like we can predict whether we're likely to get rain from looking at the colour of a cloud. Is that actually true? Peter - It's mostly true I think, because usually the ones with the bigger particles tend to appear much darker. And so, if you get the feeling over a dark, big, fat, clouds coming, that's certainly is a good sign for rain.