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Why is the sky not blue as seen from space?

Category: Earth Science
Published: October 16, 2015

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

The daytime sky is indeed blue as seen from space. Look closely at any color-accurate photograph of earth taken from space and the blue tint of everything on the day side is unmistakable. This blue tint is the sky. The daytime sky as viewed from space is not a solid, uniform blob of blue for two reasons: (1) there are white clouds in the sky which can be seen just as well from space as from earth's surface, and (2) the sky is not opaque.

earth iss
Public Domain Image, source: NASA.

Earth's atmosphere consists mostly of nitrogen molecules and oxygen molecules bouncing around as a gas. Air is close to perfectly transparent. It's a good thing, because the transparency of air is what allows you to see your computer screen and breath at the same time. However, air is not perfectly transparent. A very small amount of the light that passes through air is scattered in all directions instead of being allowed to continue traveling on in the forward direction. This scattering is called Rayleigh scattering. Blue and violet colors are scattered the strongest by air. As a result, when white light travels through air, it gives a slight bluish-white tint to the air. (Air is actually tinted violet-bluish-white. However, since humans can't see the color violet very well, we see air as bluish white.) It's true that when look at your friend across the room, you don't see her as tinted blue. But this is because there is so little air between you and your friend that the amount of light scattered by the air is far below what your eyes can detect. However, if enough air is present, the blue tint of air indeed becomes noticeable to humans. This is what the daytime sky is: a large enough mass of air that its blue tint becomes noticeable to humans.

Despite the atmosphere containing so much air, it does not contain enough air to scatter 100% of the light and therefore act as opaque. We thus see the sky as a whitish-blue semi-transparent layer. When standing on the surface of the earth and looking up at the daytime sky, you may be tempted to think that the sky is opaque. However, the daytime sky often looks like a uniform, featureless stretch of blue only because there are so few bright objects beyond the sky. The stars are so dim that their light is overpowered during the day by the sunlight scattered by the sky. The semi-transparent nature of the daytime sky is made obvious by the fact that you can see the moon during the day from the surface of the earth. And as expected, the moon as seen from the surface of the earth during the day is tinted blue. You can see the moon just fine through the daytime sky, indicting that it is semi-transparent. The atmosphere between you and the moon just causes the moon to gain a blue tint during the day. For the exact same reason, an astronaut in space that is looking down at earth's surface can see mountains and deserts just fine, but he sees them as tinted blue.

This all makes sense, but there still seems to be photos of earth taken from space that are not tinted blue. How can we account for these photos? There are a couple of reasons for this. First of all, many photos that are referred to as "space photos" or "satellite imagery" are actually taken from airplanes that are very low in the atmosphere and therefore don't pick up very much of the blue tint. For instance, most of the images in Google Earth were taken from airplanes (however, Google Earth artificially tints some images blue at certain viewing angles and zoom levels in order to make them look more realistic). Additionally, many photos of earth that are genuinely taken from space have their colors altered in order to intentionally remove the blue tint. As long as a photograph has accurate colors and is taken from space, the daytime Earth will be everywhere tinted blue because of the atmosphere.

Topics: Rayleigh scattering, blue sky, scattering, sky