Light on Gloomy Day: Why is Sky Grey?

[fouad89]

Hi

Can some one please explain to me what happens to light particles on a gloomy day? why is the sky grey? are the photons entering the atmosphere just at a low energy level?

Thanks

 

[mfb]

On a gloomy day, light is scattered significantly in the atmosphere, and a big fraction gets absorbed there, therefore less light reaches the ground.
Clouds tend to scatter light randomly, so they have a similar spectrum than sunlight - white, and "dark white" is grey.

are the photons entering the atmosphere just at a low energy level?

The photons entering the atmosphere always have the same energy/frequency distribution, as the solar radiation does not change (significantly).

 

[fouad89]

If I understand this correctly,the clouds absorb most of the incoming light spectrum? and due to scattering (and collision ?) most of the light stay inside the clouds therefore only the "white" or the "grey" part of the spectrum is seen by us? and there is no photon energy losses?

 

[sophiecentaur]

Hi

Can some one please explain to me what happens to light particles on a gloomy day? why is the sky grey? are the photons entering the atmosphere just at a low energy level?

Thanks

I'd find it very difficult to describe to you what happens to the particles of light when they enter the atmosphere because that would involve Quantum Mechanics. I am assuming that, as you asked your question in that way, you didn't want to start off with QM (?).

It would be much easier (and understandable) to describe what happens to the light Waves. (Both models - waves and particles - are equally valid). Light interacts with water droplets in the air (mist and cloud). Some of the energy in the waves is scattered and some is absorbed. This means that less arrives at the ground and the day is 'dull'. Longer wavelength light (the reds) tends to be absorbed by liquid water in the droplets (think of underwater photographs) and so there is a bit less red in the light on a dull day. The lighting appears a bit 'colder' (at least it's a colour that we associate with cold) as well as being dimmer. Thick cloud looks very dark grey.
There is another effect and that is the depth of the shadows. On a day with no cloud and the Sun high in the sky, the shadows are much deeper because the light is coming from largely one direction. Diffuse lighting due to cloud and mist 'fills' the shadows and the contrast goes down. All this is even more noticeable in photographs because our brains try to edit out a lot of what we see, in the way of colour variation etc. but even the smartest cameras don't get it perfect.

 

[mfb]

If I understand this correctly,the clouds absorb most of the incoming light spectrum? and due to scattering (and collision ?) most of the light stay inside the clouds therefore only the "white" or the "grey" part of the spectrum is seen by us?

White is not a color - it is an impression our eyes and brain can get if they see a whole spectrum, or at least multiple different wavelengths.

and there is no photon energy losses?

I don't think that is relevant in any way.

 

[fouad89]

I get it thanks

 

[haael]

Clouds block a fraction of the red light. That's why the color of the sunlight is slightly more blue. On a sunny day, you have yellow light from Sun and blue light from the sky, combining into white-yellowish color. On a gloomy day these two ranges still combine, but there is less red, so the resulting color is white-bluish.

Or perhaps you wanted to see something like that:
There is no Sun on a gloomy day. No hope. No light.

 

[sophiecentaur]

Clouds block a fraction of the red light. That's why the color of the sunlight is slightly more blue. On a sunny day, you have yellow light from Sun and blue light from the sky, combining into white-yellowish color. On a gloomy day these two ranges still combine, but there is less red, so the resulting color is white-bluish.

Or perhaps you wanted to see something like that:
There is no Sun on a gloomy day. No hope. No light.

A bit of an over-simplification here, I think. Light from the 'blue' of the sky is, in fact, very de-saturated and there is a lot of longer wavelength light in there too (it's pretty much white as an illuminant). The light, direct from the Sun is regarded as 'white' and corresponds to a temperature of about 5800K. It's the 'whitest' illuminant you can get without using filters and discharge lamps - the hottest Tungsten filament is more yellow (play with the colour balance controls on a good quality camera to see the differences). Moreover, the light from the whole of a clear blue sky only contributes about 10% or less to the illumination (note the depth of the shadows under these conditions and that colours do not look noticeably different from in the direct sunlight). Actually, it's difficult to get really cloudless and haze-less skies in the UK to find the deepest shadows possible - where light only comes from the 'blue sky'. Cameras give odd measurements because the exposure and black level setting tend to compensate for the actual contrast ratio. Our eyes are worse still as they (plus the brain) do their very best to eliminate such things on the way to our conscious view of a scene.

 

[fouad89]

what if we'r talking outside the visible spectrum, say IR or UV would those wavelengths hit the surface of the earth?

 

[sophiecentaur]

Yes but with less intensity - The spectrum of sunlight peaks in the visible region. There is a bit of a 'window' through the atmosphere in the visible region. Some IR gets through but a lot is absorbed (reason for the greenhouse effect is that IR from the hot ground is absorbed and re-radiated by the lower atmosphere). UV is absorbed on the way through but, of course, some gets down here and even more on mountain tops.

We are lucky with the range of wavelengths that penetrate in that the harmful stuff is largely filtered out and our sensors work best in the region where most energy arrives. That's evolution at work as well, of course but IR photons carry less energy and not all sensor technologies are as sensitive - as it happens, solid state sensors are v sensitive to some IR wavelengths so cameras need filters in front of the sensors to remove IR.

 

[fouad89]

would the IR or UV spectrum penetrate through a cloud of dust ?

 

[sophiecentaur]

No more than the light would.