Why does the sky sometimes appear white?

The sky sometimes appears white because our atmosphere is comprised of gas molecules such as oxygen, nitrogen as well as dust particles. The size of these molecules & atoms differ to a large extent. Also there is water vapour in the atmosphere whose size is much larger compared to the gas molecules. So what happens is, theses larger particles of dust & molecules of water vapour scatter the colours of other frequency except blue as the size of the atoms of dust & water molecules are not comparable with the wavelength of blue light. So the scattered light of all the other frequencies gets combined and reach our eyes and thus the sky appears white.