Why Is the Sky Blue at Noon but Red at Sunset?

1. Sunset Red Is What Is Left After the Blue Has Gone

A red sky looks as if someone has painted it. But the explanation goes the other way. At sunrise and sunset the Sun is low in the sky, so its light has to pass through a thicker layer of atmosphere, and the shorter-wavelength blue light gets scattered away. What reaches our eyes from the direction of the Sun is mostly the longer wavelengths: reds and oranges[1].

"Scattered" means knocked sideways: the light bounces off tiny bits of air and heads off in a new direction. So the red in the evening sky is not new. It was in the sunlight all along, and it is simply what remains when the blue has been taken out of the beam.

That raises two smaller questions. Why does air knock the blue sideways so much more than the red? And why does a low Sun mean more of it is lost?

2. Air Molecules Scatter Blue Light Far More Than Red Light

Sunlight looks white, but it is a mix of colors, and each color is a wave of a different length. Blue light has shorter waves than red light. NASA's Space Place says blue light is scattered more than the other colors because it travels as shorter, smaller waves[2].

Physicists call this Rayleigh scattering. It applies when the scattering particles, here the molecules of the air, are much smaller than the wavelength of the light. The amount of scattering is inversely proportional to the fourth power of the wavelength[3]. That is a steep rule. As a rough example, take red at 700 nanometers and blue at 450 nanometers (typical values; the exact numbers depend on which shades you pick). Dividing and raising to the fourth power gives about 5.9, so blue light is scattered about six times as strongly as red. This is our own arithmetic from the rule, not a figure from the sources.

Those scattered blue rays go off in all directions. That is the daytime sky: when you look at a part of the sky away from the Sun, blue light that has been knocked sideways is arriving from there[2][1].

3. A Low Sun Sends Its Light Through More Air, So More Blue Is Lost

At noon the Sun is high, and its light crosses the atmosphere by a fairly short route. Near the horizon, the same light crosses far more air before it reaches you. NASA's Space Place describes the effect: as the Sun gets lower, the light passes through more atmosphere, so blue is scattered away again and again, while the longer red and yellow wavelengths get through more easily[2]. Royal Museums Greenwich puts it the same way: with the Sun low, the light travels a longer distance and the blue is scattered away[4].

This is why the two colors belong together. The blue that is missing from the sunset beam has not vanished. It was scattered into the sky, and it is part of the blue light that people elsewhere, or at other moments, see overhead. Blue sky and red sunset are the same scattering, seen from two directions: the light that was knocked aside, and the light that went straight through. (That last link between the two is our own way of putting together what the sources say.)

None of the sources we read gave a number for how much longer the route is near the horizon, so we do not give one either.

4. Violet Light Scatters Even More, So Why Is the Sky Not Violet?

Violet has shorter waves than blue, so by the rule above it should scatter even more. Yet nobody calls the sky violet. Two reasons are given. There is less violet than blue in the sunlight that reaches the top of our atmosphere, and our eyes are more sensitive to blue[4][1].

So the color of the sky is not decided by physics alone. It depends on how much of each color the Sun sends out, how strongly the air scatters it, and how well our eyes respond to it.

5. On Mars, Fine Dust Makes the Sky Around the Setting Sun Blue

If a long path through air removes blue, you might expect every sunset on every planet to be red. On April 15, 2015, NASA's Curiosity rover, working near the Martian equator, took a color picture sequence of a sunset lasting about seven minutes. The area around the Sun looks blue[5]. The images were taken after a dust storm had swept over the rover's site[6].

NASA's explanation is that Martian dust lets blue light through more efficiently than red, the opposite of what air molecules do on Earth[5]. A member of the Curiosity team, Mark Lemmon, added the detail that the very fine dust is the right size so that blue light scatters off it but stays closer to the direction of the Sun than other colors do. The rest of the sky is yellow to orange[6].

The lesson carries back to Earth. What color you see depends on what the particles are and how big they are, and on which direction you look. A small change in the particles can turn the same idea, light bouncing off tiny things, into a very different sky.

6. Two Ways to Look for Sunset Colors Yourself

A glass of milky water

Fill a tall clear glass with water and stir in a little milk. Darken the room and shine a flashlight through the glass. A classroom activity from the University Corporation for Atmospheric Research describes the result: the beam looks blue where it has crossed a short stretch of milky water, and looks yellow, orange, or red at the far end after it has crossed most of it[7]. If the glass looks too clear or too cloudy, change the amount of milk. Do not point the flashlight at anyone's eyes.

The evening sky

On a clear evening, compare the sky near the Sun with the sky on the opposite side. Never look straight at the Sun. Face away from it, or hold up a hand to block it, and look only at the sky around.

Sources

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