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Most of us here are aware as I think that pure water has a natural quest to absorb visible red colour wavelength slightly due do which liquid water and sometimes even ice appears to have a blue colour tinge.

But even gaseous water(water vapour or steam) also absorbs red, rather it absorbs more strongly the red then liquid and solid water due to absence of hydrogen bonding.

So why don't clouds/ masses of suspended water vapour appear blue. Why do clouds simply scatter/reflect the white light that falls upon them instead of simultaneously absorbing the red from white llight along with scattering of white light and finally appearing blue just like water.

The proof of gaseous water absorbs red more strongly then liquid and solid water - http://www.webexhibits.org/causesofcolor/5B.html In this link look up in "What is the role of hydrogen bonding" section.

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The white colour of clouds is due to Mie scattering. This arises because the refractive index of water is different from air. If the particles are large enough all wavelengths are scattered equally so there is no change in the colour of the scattered light. This is the case in clouds where droplet sizes are typically 10 to 20 microns.

The light scattering is from the surface of the drops i.e. the air/water interface. There will be some light transmitted through drops and hence some red absorption, but this is small compared to the Mie scattering so any colour shift is too small to see. Nevertheless the light colour does shift to the blue on cloudy days. Photographers judge the light colour by the colour temperature. Regular sunlight is around 5,700K and temperatures higher than this are bluer while lower temperatures are redder. On cloudy days the colour temperature is higher so the light is bluer. Google for related articles - a quick Google found this article that seems to me to give a good overview.

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  • $\begingroup$ Thanks Mr. John Rennie for the extraordinary explanation. I am almost clear now. Just one more question which is propping up in my mind… As you said,there will be some light entering the water droplets so some of the red is absorbed.but as this effect is too small compared to mie scattering and so is often negligible and finally clouds appear white. But now if I say I have a very big giant chamber filled with dry steam (no other air particles) and if I shine white light into it will the red coloured wavelength be absorbed by the steam and so consequently will it appear blue??? $\endgroup$ – user38220 Feb 26 '14 at 12:42
  • $\begingroup$ Continued @ John Rennie- Also assume that as the steam is dry there are no big water droplets to cause mie scattering. And if it does appear blue then is it due to- 1) The preferential absorption of red or 2) Is it due to the reason that higher frequency blue is scattered more compared to red light(The same reason Why is sky blue.) or 3) Or will it appear blue due to combination of both the effects,namely the preferential absorption and the latter one namely Rayleigh scattering. Once again a hearty thanks for your kind attention and the fabulous link. Sorry for the long comment. $\endgroup$ – user38220 Feb 26 '14 at 12:44
  • $\begingroup$ @user38220: The only spectrum of water vapour I could find is here. From this it looks as if the absorption starts around 750nm, which is out of the visible range. So water vapour would not have a blueish colour. It's hard to be certain though without a better quality spectrum. $\endgroup$ – John Rennie Feb 26 '14 at 15:04
  • $\begingroup$ -But won't atleast lots of pure dry steam(no water droplets, only gaseous water) appear blue in white light by Rayleigh scattering phenomenon in which higher frequency blue light is scattered more compared to low frequency red light (The same reason why sky is blue).... $\endgroup$ – user38220 Feb 26 '14 at 17:58
  • $\begingroup$ @John-I really appreciate your efforts in helping me to clear out...Thanks very much. $\endgroup$ – user38220 Feb 26 '14 at 18:01

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