Blue Sky, Blue Sea
October 15, 2002
On the photo above, taken from the harbor in Khania, Greece (Crete), the color of the Mediterranean Sea appears a deeper blue than that of the sky. On a clear day, the blue color of the Mediterranean and many other seas can be striking. Pure water has an intrinsic pale blue color, which is neither due to scattering (like the sky), nor to dissolved impurities. Water absorbs most of the red wavelengths of light, but its peak transparency in the blue-green part of the spectrum. Thus, when observing light that has passed through several meters of water, the colors we see are the wavelengths of light that aren't as readily absorbed, namely blue and green. Additionally, the color of the sea results from a number of other factors, including how deep it is, reflections from the sky and the smoothness of the sea surface. On an overcast day, the sea won't look as blue as it does on a clear, sunny day.
On the other hand, the color of the sky is a result of molecules of air preferentially scattering the shorter blue wavelengths of incoming sunlight. Note on the photo above, that the sky near the horizon is rather bright. The reason for this is that scattering is greater here since sunlight reaching our eyes from the direction of the horizon must travel through more of the atmosphere than light that approaches our eyes directly overhead.