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Why is ice more reflective (has higher albedo) than liquid water? They're both the same substance (water). Is something quantum mechanical involved?

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In fact ice is slightly less reflective than water. The reflectivity is related to the refractive index (in a rather complicated way) and the refractive index of ice is 1.31 while the refractive index of water is 1.33. The slightly lower refractive index of ice will cause a slightly lower reflectivity. In both cases the reflectivity is about 0.05 i.e. at an air/water or air/ice surface about 5% of the light is reflected.

Water generally has a relatively smooth surface so the light falling on the water only gets a chance to reflect back once. Any light that doesn't reflect off the surface propagates down into the water where it is eventually absorbed and converted to heat. The end result is that a large body water reflects only about 5% of the light.

Ice is generally covered with some snow, and snow is made up of small ice crystals with air gaps between them. Light falling onto snow may be reflected at the first surface, but any light that isn't reflected will meet lots more ice/air interfaces as it travels through the snow, and at every surface more light will be reflected. The net result is that much more of the light is reflected from snow.

So the difference isn't anything fundamental, it's just because water is continuous while snow isn't. It is possible to form an air water dispersion, for example foam or fog. Both foams and fogs reflect light far more efficiently than a large body of water.

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  • $\begingroup$ That doesn't jibe with the given albedos for sea water and sea ice, for example nsidc.org/cryosphere/seaice/processes/albedo.html . According to it, ice is way more reflective than water. $\endgroup$ – DrZ214 Jul 11 '15 at 8:45
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    $\begingroup$ Sea ice has lots of air bubbles trapped in it, and you get multiple scattering from all those air/water interfaces. If you prepare ice carefully and exclude all impurities the result are ice sheets as transparent as glass. $\endgroup$ – John Rennie Jul 11 '15 at 9:47
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    $\begingroup$ To see what John Rennie is talking about, watch youtube.com/watch?v=8WBqX7MSqWw for an example of naturally-occurring clear ice. Also note that even at shallow angles the reflectivity is comparable to liquid water. $\endgroup$ – WhatRoughBeast Jul 11 '15 at 17:32
  • $\begingroup$ Then can I just ask how air bubbles would make it drastically more reflective? Since air by itself is much less reflective than water, isn't it? $\endgroup$ – DrZ214 Jul 11 '15 at 18:52
  • $\begingroup$ @DrZ214: at every air/water boundary about 5% of the light is reflected and 95% is transmitted. Suppose a typical ray of light passes through $n$ boundaries, the fraction reflected is of order $1 - 0.95^n$. The presence of lots of bubbles means $n$ is large and the fraction reflected is close to one. $\endgroup$ – John Rennie Jul 11 '15 at 19:22

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