Great Uncle Fred, why is the sky blue on a cloudy day? (Anonymous Questioner, Autumn 2008)

It`s not. It`s been painted so many times as blue, whatever the weather, that people think that it is blue. Like trees, which are not generally brown, even though tables, it is true, often are brown. Trees, on the whole, are grey, as is the sea on a cloudy day. The sea reflects the colour of the sky which is, admittedly, often blue. The sky is blue because the light from the sun, which looks white but is made up of all the colours of the rainbow, comes in different lengths of wave (red is the longest, violet the shortest) and when it reaches the Earth`s atmosphere and gets scattered all around by the trillions of molecules, the shorter waves are scattered more and are therefore seen more. There is less violet light and our eyes are designed to respond most strongly to red, green and blue light; otherwise the sky might look violet. The sea scatters the blue light as well, and so looks extra blue, but only on a sunny day. On a cloudy day, it looks rather similar to the colour of a cloud. Unglamorous, but true.

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