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Oliver Walker
Oliver Walker

Skies Are Blue



Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.




skies are blue


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Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions.


As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes.


As seen from space, planet Earth is often described as a pale blue dot, but it's only the oceans that appear blue-hued. The continents, clouds, and ice caps don't appear blue at all; it's the oceans, not the atmosphere, that give our planet its overall complexion. For thousands of years, humanity had to simply accept these properties of our world as facts. But with the advances of modern science, we understand why both the skies and oceans are blue.


Contrary to what you might have read, there's no one single factor responsible for Earth's blue skies. The skies aren't blue because sunlight has a blue tint; our Sun emits light of many different wavelengths, and that light sums up to be a net white color. Oxygen itself isn't a blue-colored gas, but rather is transparent to light. However, there are a myriad of molecules and larger particles in our atmosphere that do play a role, scattering light of different wavelengths by different amounts. The ocean plays no role in the color of the skies, but the sensitivity of our eyes absolutely does: we do not see reality as it is, but rather as our senses perceive it and our brain interprets it.


The violet light at the short-wavelength limit of what we can see scatters over nine times more frequently than the red, long-wavelength light at the other end of our vision. This is why, during sunrises, sunsets, and lunar eclipses, red light can still pass efficiently through the atmosphere, but the bluer wavelengths of light are practically non-existent, having been preferentially scattered away.


Since the bluer wavelengths of light are easier to scatter, any incoming direct sunlight will become redder and redder the more atmosphere it passes through. The remainder of the sky, however, will be illuminated by indirect sunlight: light that strikes the atmosphere and then gets redirected towards your eyes. The overwhelming majority of that light will be blue in wavelength, which is why the sky is blue during the day.


It will only take on a redder hue if there's enough atmosphere to scatter that blue light away before it reaches your eyes. If the Sun is below the horizon, all the light has to pass through large amounts of atmosphere. The bluer light gets scattered away, in all directions, while the redder light is far less likely to get scattered, meaning it takes a more direct path to your eyes. If you're ever up in an airplane after sunset or before sunrise, you can get a spectacular view of this effect.


This might explain why sunsets, sunrises, and lunar eclipses are red, but might leave you wondering why the sky appears blue instead of violet. Indeed, there actually is a greater amount of violet light coming from the atmosphere than blue light, but there's also a mix of the other colors as well. Because your eyes have three types of cones (for detecting color) in them, along with the monochromatic rods, it's the signals from all four that need to get interpreted by your brain when it comes to assigning a color.


Each type of cone, plus the rods, are sensitive to light of different wavelengths, but all of them get stimulated to some degree by the sky. Our eyes respond more strongly to blue, cyan, and green wavelengths of light than they do to violet. Even though there's more violet light, it isn't enough to overcome the strong blue signal our brains deliver, and that's why the sky appears blue to our eyes.


The oceans, on the other hand, are an entirely different story. If you take a look at the planet as a whole, with a view such as the one you get from space, you'll notice that the bodies of water we have aren't a uniform blue, but rather vary in their shade based on the water's depth. Deeper waters are a darker blue; shallower waters are a lighter blue.


The farther down we go, as we reach depths of 30 meters, 100 meters, 200 meters and more, the bluer everything appears. This makes a lot of sense when you remember that water, just like the atmosphere, is also made out of molecules of a finite size: smaller than the wavelengths of any light that we can see. But here, in the depths of the ocean, the physics of scattering is a little different.


This is why the deepest ocean depths appear a deep, dark blue: because all the other wavelengths get absorbed. The deepest blues, unique among all the wavelengths of light in water, have the highest probability of getting reflected and re-emitted back out. As it stands, the global average albedo (the technical term for reflectivity) of our planet is 0.30, meaning 30% of the incident light gets reflected back into space. But if the Earth were entirely deep-water ocean, our albedo would be just 0.11. The ocean is actually pretty good at absorbing sunlight!


For the skies, the blue sunlight scatters more easily, and comes to us indirectly from where sunlight strikes the atmosphere as a result. For the oceans, longer-wavelength visible light gets absorbed more easily, so the deeper they go, the darker bluer the remaining light appears. Blue atmospheres may be common for planets, as Uranus and Neptune both possess them, too, but we're the only one we know of with a blue surface. Perhaps when we find another world with liquid water on its surface, we won't be so alone in more ways than one!


We might expect it to have a very faint blue coloured sky, but due to the haze of dust that remains suspended in the air the daytime sky on Mars appears more yellow. This is because the larger dust particles absorb the short wavelength blue light, and scatter the remaining colours to give a butterscotch hue over the Martian sky.


This same sort of equal-opportunity irreverence is on abundant display in Tom Scocca's Beijing Welcomes You: Unveiling the Capital City of the Future . It's a spirited portrayal of an old metropolis being turned inside out for the 2008 Olympic Games and, by his own account, a clueless young American writer's effort to make sense of the situation. The author toggles between describing maddening things about Beijing life (lightbulbs that inexplicably explode, polluted air that gives his newborn asthma, happy-talk propaganda that pretends that obviously smoggy skies are blue, etc.) and pointing out how outlandish the behavior and beliefs of Americans, himself included, can seem to residents of China's capital. (See pictures of Beijing's Transit System.)


My favorite section, which brought both Twain and Dave Barry to mind, involves a run-in with airport security in Xinjiang. Scocca and his wife are told they can't board their Beijing-bound plane without surrendering two jars of baby food that they've gone to great lengths to acquire for their newborn. The crisis escalates until his wife wrests a confiscated jar of strained berries from a guard and starts spooning fruit "into the baby's mouth," only to find that even this won't convince him the product is harmless. "Unimpressed, the officer held on to the second jar," Scocca writes, assuming that "the blueberries were a decoy, and the strained peas were the real explosives." (See pictures of the Beijing Olympics.) 041b061a72


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