why is the sky blue

[asset189]

When looking up, yet when looking down from the ISS live feed its basically transparent? Sure there is a simple explanation

[ScottS]

It not. Not where I am anyway. More a dull grey.

[ronin]

The Red component of sunlight sort of hits the air and goes on straight.

The Green bit is simialr but some of it gets bounced around.

The Blue bit gets bounced around a lot.

Bounced around = Scattered.

So when you look at a patch of sky the light from it has in effect a lot of Blue, a bit of Green and not much Red.

So you see Blue, the other bits make it a sort of lightish blue. R+G+B = White.

Scattering is a function of frequency to the fourth power.

If Red = 1 then Blue = 2 and green falls in the middle at 1.5 (Ratioo's of frequencies)

So from a patch of sky you get 1⁴ of red = 1, 1.5⁴ of green = 5 and 2⁴ of blue = 16.

So your Blue sky is about 16 bits blue, 5 green and 1 red.

What you have to remember is that the light is NOT direct from the sun and without scattering the sky would be black, and you could possibly see stars during the day.

Using the same arguement work out why a sunset is Red. :eek:

All figures are a reasonable approximation.

[ScottS]

What you have to remember is that the light is NOT direct from the sun and without scattering the sky would be black, and you could possibly see stars during the day.

As per images from the moon (at any time).

[ollypenrice]

Writing at the same time as Ronin, this is what I'd said;

The terrestrial situation: short wavelength (blue) light is scattered far more readily by the atmosphere than light of longer wavelengths so instead of coming straight from the sun to our eyes it goes flitting about through the atmosphere before eventually and randomly coming to our eyes from wherever it happened to have been scattered to. So the sky looks blue. (It doesn't look blue for tha same reason that a Tory rosette looks blue. A Tory rosette absorbs all but blue light and only reflects the blue. Not the same situation.)

 

Ah, now for the hard part! I haven't been on the ISS ( ) so all of this will be conjecture. For us to descirbe the atmosphere as looking transparent that means we must be seeing sunlight which has passed through the atmosphere and reflected back to our eyes from the earth. Different parts of the earth's surface reflect different colours so we feel we see it quite well - provided there is no cloud cover. But why doesn't it look blue? Good question! FIrstly are we sure the filters used in the images haven't been compensated? Secondly, could it be that up at the ISS there is not sufficient atmosphere in the vicinity to bring the scattered blue light to our eyes? Does more of it escape into space in directions away from our eyes? Hmmm....

 

The second part of the question is quite tricky! Watching with interest.

 

Olly

[orion25]

Good question! It's because the ISS orbits higher (about 400 km) than the part of our atmosphere that looks blue. If you closely at pictures from the ISS, you may notice a tight blue halo around the Earth. That's the blue of our atmosphere. The ISS is in the thermosphere, the second highest layer of our atmosphere, almost into outer space. The atmosphere gets thinner and thinner as you go up, eventually fading into outer space. That's why the sky looks black when looking down at Earth from ISS.

Reggie 

[Knight of Clear Skies]

Ah, now for the hard part! I haven't been on the ISS ( ) so all of this will be conjecture. For us to descirbe the atmosphere as looking transparent that means we must be seeing sunlight which has passed through the atmosphere and reflected back to our eyes from the earth. Different parts of the earth's surface reflect different colours so we feel we see it quite well - provided there is no cloud cover. But why doesn't it look blue? Good question! FIrstly are we sure the filters used in the images haven't been compensated? Secondly, could it be that up at the ISS there is not sufficient atmosphere in the vicinity to bring the scattered blue light to our eyes? Does more of it escape into space in directions away from our eyes? Hmmm....

 

Now you've got me thinking. The Moon can appear quite blue when viewed during the day...

But then the Moon has a low albedo, notice how the darkest areas of the Moon appear the bluest in the image above. The brighter areas appear almost white but their albedo is still quite low compared to most things down here. So I'd guess it's simply a case that when looking down from the ISS any scattered light from the atmosphere will be overwhelmed by the much brighter light reflected from the surface, so any blue tint isn't obvious.

The belly of the plane is blue for another reason known as BA.

[orion25]

This NASA ISS image is a good example of how our blue atmosphere looks from that vantage point:

 http://spaceflight1.nasa.gov/gallery/images/station/crew-30/html/iss030e031275.html

[D4N]

The reflected light from the moon is also scattered and interfered with by scattered sunlight.

It looks red when I view it though as I use a filter

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[SkySurfer]

So all in all NO there is no simple explanation........lol

:)

SS

[ollypenrice]

So all in all NO there is no simple explanation........lol

:)

SS

Simple astronomy? Ahem, you might just as well ask for simple love or simple tax returns.

lly

[Ptarmigan]

When looking up, yet when looking down from the ISS live feed its basically transparent? Sure there is a simple explanation

Good question !

I think it is all to do with the backgroud to the blue that one is looking at and the thickness of the atmosphere.

You can pretend you are on your own ISS by looking down upon the moon, iyswim, ??

It does not look blue. Well not often !!

The sky only looks blue because the small amount of blue scattered light is overlaid on the black background of space, there isnt much (significant) of it only a mile or two and no back lighting to confuse the view ?

But go to the top of a mountain and look across at a some more mountains way over on the horizon, they are starting to look a bit blueish with 20 or 25 miles of atmosphere getting in the way.

Which is why photographers fix their UV filters on the front.

[ollypenrice]

Good question !

I think it is all to do with the backgroud to the blue that one is looking at and the thickness of the atmosphere.

You can pretend you are on your own ISS by looking down upon the moon, iyswim, ??

It does not look blue. Well not often !!

The sky only looks blue because the small amount of blue scattered light is overlaid on the black background of space, there isnt much (significant) of it only a mile or two and no back lighting to confuse the view ?

But go to the top of a mountain and look across at a some more mountains way over on the horizon, they are starting to look a bit blueish with 20 or 25 miles of atmosphere getting in the way.

Which is why photographers fix their UV filters on the front.

'Those blue-remembered hills...'

lly

[Ptarmigan]

 notice how the darkest areas of the Moon appear the bluest in the image above.

You have me thinking as well !

The darkest bits goes along with my thoughts on a black background to view the blueness

Another thing that comes into play with photographs is - were you using auto-white-balance ?

Interesting. I'll be taking a closer look at the moon when next it is in a blue sky, I'm sure I remember it yellower than that !

[Ptarmigan]

'Those blue-remembered hills...'

lly

:)

Is that why hill-billies write blue grass songs ?

Why do poets always get the worms all muddled up ?

Did he mean "those blue hills that I remember"

Or is he remembering some hills in a sad way or mood, like with a dose of 'the blues' ?

[Knight of Clear Skies]

The sky only looks blue because the small amount of blue scattered light is overlaid on the black background of space, there isnt much (significant) of it only a mile or two and no back lighting to confuse the view ?

You have me thinking as well !

The darkest bits goes along with my thoughts on a black background to view the blueness

Another thing that comes into play with photographs is - were you using auto-white-balance ?

We seem to be thinking along the same lines. Good point about auto-white balance though, it was on so my image could be a little deceptive. I probably boosted the saturation as well.

[asset189]

Begining to wish id never asked he he. But seeing as every ones brain is working hard, the same blue sky looking up becomes transparent at night - why?

[D4N]

Because the sun is being blocked by the Earth, no direct sunlight to scatter in the atmosphere.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[michael8554]

It's so obvious guys.

Slartibartfast looked at the concept photos and said "no no no, I said BLUE sky".

Simples.

[Carina Lass]

"Why is the sky blue"?

This is telling you that you should set your telescope up for an evening under the stars.

It is also telling you that you are a mug.

As the sky will go cloudy just before you complete your PA.

[Qualia]

Light comes in a whole load of different wavelengths, but which wavelengths correspond to which colour depends on the eyes of the creature doing the looking, and not on any property of the light itself. As such, there isn’t any objective real colour in the world. The colour of the morning sky, the colour of the rainbow and of my sweet heart's lips is nothing more than an illusion.

When you see a blue sky, a given wavelength on the electromagnetic scale is being scattered, and these light waves correspond to what we call blue. But that doesn’t mean that the light itself is blue, it just means that a human brain equipped with human eyes will label it as blue. If you were colour blind, it could be a shade of green; if you were a bird or insect, you'd probably see some ultra-violet hue of blue.

[ollypenrice]

Begining to wish id never asked he he. But seeing as every ones brain is working hard, the same blue sky looking up becomes transparent at night - why

Because the sun is now illuminating a different part of the globe. Why doesn't it go dark the instant the sun sets, ie why does the blue sky malinger after sunset? Because rays from the set sun are refracted into the atmosphere and curved to reach us for a while longer. The blue scattering continues in the upper atmosphere. The sun needs to be 18 degrees below the horizon for this to cease.

Olly