Why Is the Sky Blue?

What colors make up sunlight?

Sunlight looks white, but it is really a blend of all the colors of the rainbow.^[1] Isaac Newton demonstrated this with a prism, which separated the colors into a spectrum.^[4]

Light travels in waves, and each color has its own wavelength, the distance from one wave crest to the next.^[2] Red has the longest wavelength in the visible spectrum, and violet has the shortest.^[2] NOAA puts a red light wave at about 750 nanometers and a blue or violet wave at about 400 nanometers.^[3] A nanometer is one-billionth of a meter, and a human hair is about 50,000 nanometers thick.^[3] Sources round the limits slightly differently: the Usenet Physics FAQ gives about 720 nm for red and about 380 nm for violet.^[4]

• Red light750 nm
• Blue or violet light400 nm

Source: NOAA National Environmental Satellite, Data, and Information Service (NESDIS) [3]

How does Rayleigh scattering make the sky blue?

Light moves in a straight line until it hits something in its path, such as a speck of dust or a gas molecule of oxygen or nitrogen.^[2] These gas molecules are smaller than the wavelengths of visible light, and although they scatter every color, they scatter violet and blue the most.^[2] Much of the red, yellow and green light passes straight through the atmosphere, while the blue and violet waves are sent off in every direction.^[3] With violet and blue light scattered across it, the sky appears blue.^[3]

The process is called Rayleigh scattering, after Lord Rayleigh, who, according to the National Weather Service, first described it in the 1870s.^[2] The Usenet Physics FAQ traces the first steps to John Tyndall, who found in 1859 that small particles suspended in a clear fluid scatter blue light more strongly than red.^[4] Rayleigh studied the effect in more detail and showed that, for small enough particles, the amount of scattered light is inversely proportional to the fourth power of the wavelength.^[4] The Exploratorium puts the difference at approximately ten times more scattering for blue light than for red.^[6]

Tyndall and Rayleigh thought that dust and droplets of water vapor in the air caused the blue color.^[4] Later scientists concluded that the oxygen and nitrogen molecules of the air were enough to explain it.^[4] Einstein later settled the question by calculating a detailed formula for light scattered by molecules, and the formula agreed with experiment.^[4]

Why are clouds white and the horizon pale?

Rayleigh scattering applies to air molecules and to particles up to about a tenth of the wavelength of the light.^[5] The water droplets in a cloud are much larger than air molecules and scatter all visible wavelengths almost equally, which is why clouds look white or gray.^[5]

Near the horizon, the blue gives way to a lighter blue or white.^[1] Light from low in the sky has crossed more air than light from overhead, and the added scattering blends the colors back together.^[1]

Why is the sky blue and not violet?

Violet light has an even shorter wavelength than blue light, and it is scattered even more.^[6] The sources give three reasons the sky still looks blue, and they do not weigh them the same way.^[2][3][4][6]

• The Sun emits more energy as blue light than as violet light.^[2][6] The Exploratorium gives this as its only reason: there is just not enough violet.^[6]
• Some violet light is absorbed by the upper atmosphere.^[3][4]
• Human eyes are less sensitive to violet than to blue.^[2][3][4]

The Usenet Physics FAQ treats the first two points as only part of the answer and says the rest lies in how vision works.^[4] The retina has three types of color receptors, called cones, which respond most strongly to red, green and blue light.^[4] Light from the sky stimulates the red and green cones about equally and the blue cones more strongly, a combination that accounts for the pale blue color people see.^[4]

Why are sunsets and sunrises red?

When the Sun is low in the sky, its light passes through more of the atmosphere before it reaches an observer.^[1][2] The longer path holds more molecules, which scatter more of the violet and blue light.^[2] Over a long enough path, all of the blue and violet is steered away from the viewer, while much of the pink, orange and red light continues straight on.^[2]

In clear air a sunset appears yellow, and small particles in the air make it redder.^[4] The sources describe the role of particles differently.^[1][3] NASA's Space Place says small particles of dust, pollution or other aerosols scatter blue light as well, so the light that gets through is more purely red and yellow.^[1] NOAA's page instead points to larger particles of dust, pollution and water vapor, which it says reflect and scatter more of the reds and yellows and can make the whole western sky glow red.^[3] The National Weather Service adds that the most spectacular sunrises and sunsets can occur after a major volcanic eruption.^[2]

What color is the sky on Mars and the Moon?

Sky color depends on what is in the atmosphere.^[1] The atmosphere of Mars is very thin, consists mostly of carbon dioxide and carries fine dust, which does not scatter light the way the gases and particles of Earth's atmosphere do.^[1] During the day the Martian sky is orange or reddish, but at sunset the sky around the Sun turns blue-gray, the opposite of the pattern on Earth.^[1]

A NASA caption for a Martian sunset image taken by the Imager for Mars Pathfinder explains both colors.^[7] Martian dust absorbs blue light, which makes the sky red, and because of its size it also scatters some blue light into the part of the sky just around the Sun.^[7] The blue shows only near sunrise and sunset, when sunlight crosses the largest amount of dust.^[7]

The Moon has no thick atmosphere to scatter blue light, which leaves its daytime sky black.^[8] On Earth, the sky directly overhead can look very dark blue or even bluish-violet at high elevations, where fewer molecules scatter light.^[2]