B Why do oceans look blue from orbit?

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This is something that one seldom stops to think about, but I suddenly thought of it myself: Why do oceans look deep blue when photographed from orbit?

Oceans look blue when looked from the shore because they are reflecting the sky... But the atmosphere doesn't look deep blue when photographed from orbit! Surely if the atmosphere were deep blue when looked from this perspective, then eg. clouds would be heavily blue-tinted, yet they are white in all the photographs (with, perhaps, at most a very, very slight and faint bluish tint to them). Also all the land would look like looked through a blue filter.

If oceans are reflecting the sky and that's why they look deep blue from orbit, that would mean that the atmosphere looks deep blue only when looked from one side, but not the other. This seems incomprehensible.
 

sophiecentaur

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The reflectivity of the ocean is quite low (compare the clouds and the water in typical images) because most of the light energy is absorbed; what you see is the result of different levels of absorption for different wavelengths. Longer wavelengths are absorbed in water so the light that's backscattered from the oceans will be predominantly blue.
The atmosphere itself doesn't absorb much light so any effect is slight. The blue sky that we see from Earth is, in fact, not 'very blue'; it has plenty of the longer wavelengths in it (Research Rayleigh scattering) but, psychologically, we see the bluish colour as a strong blue.
the atmosphere looks deep blue only when looked from one side, but not the other.
Our appreciation of the colour is based on the context. Viewing from the surface, we see the sky with black space behind it. Viewed from space, the background (land and sea) is bright and it swamps any sensation of colour from the whispy atmosphere. This is a bit like the way a one way mirror works although it transmits the same amount of light both ways.
 
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Andy Resnick

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This is something that one seldom stops to think about, but I suddenly thought of it myself: Why do oceans look deep blue when photographed from orbit?
Raman scattering.

 

sophiecentaur

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Raman scattering.

The loss of Red through water must have some effect. Would it not be measurable, compared with the scatter?
 

gleem

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When you are up close to the ocean as on a boat some waters have a define purple hue. Consider Homer's "wine dark seas"
 
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So, water looks blue, because it's blue. Got it.
 

phinds

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So, water looks blue, because it's blue. Got it.
Uh ... if you fill a shallow pan with either fresh water or saltwater would you expect to see any blue?
 
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If there was enough of it!
 
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Uh ... if you fill a shallow pan with either fresh water or saltwater would you expect to see any blue?
A block of ice looks blue, but not an ice cube...
 

sophiecentaur

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So, water looks blue, because it's blue. Got it.
That's as good a reason as any. Colour is totally subjective or we wouldn't be impressed by the feeble colours on a rainbow or a blue sky or the 'red' sunset. They would all measure as low saturation colours.

You don't need to dive to much of a depth in order to see the way red wavelengths are absorbed preferentially.
 

pinball1970

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A block of ice looks blue, but not an ice cube...
If it looks blue the object is reflecting bluer wavelengths into your retina. The pigments there are more responsive in this wavelength.
From space? Same thing I guess, blue green reflected redder longer absorbed
 

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