Science Facts

Why Is The Sky Red At Night, Sunrise & Sunset?

Red Sky Explanation

Sunlight looks bright and clear, but it’s made of multiple colors called the color spectrum, and they combine to produce white light. The sunlight or white light is a mix of all the colors of the rainbow. A rainbow is the visible colors of the spectrum, all separated. Each color in the spectrum has a different wavelength. Some light travels in short waves, and other light travels in longways.

Also, light travels from the sun to the earth. It has to travel through the Earth’s atmosphere. The atmosphere we live in, called the troposphere, comprises oxygen, nitrogen, and other molecules like water vapor.

At sunrise, night, and sunset, the Sun is close to the horizon. So the sunlight is traveling through a large distance in the Earth’s atmosphere. Red light is scattered the least, blue and green light get scattered more. Due to the large wavelength, red light scatters less. So it reaches our eyes, and that’s why the Sun appears red during sunset and sunrise.

Why is the sky red at night, sunrise & sunset?

The light from the Sun enters our atmosphere and is scattered about air. The air is filled with nitrogen, oxygen, argon, water vapor, and particles like smoke, ash, pollution, etc. These gases serve as filters or prisms to give us different types of light. The rainbow of colors that make up light blue and purple are scattered.

Scientific Explanation: The sun is closer to the horizon, which means that the light has to travel for a greater distance from the atmosphere. Long wavelengths give us the red and orange colors, where short wavelengths of light give us the blue and indigo colors. When sunlight hits these atmospheric molecules, it bounces off them and gets scattered in all directions. It’s called Reilley scattering.

The scientific reason is they have the shortest wavelength. The other colors can make it passes all those bumps in the atmosphere. The bumps being nitrogen, oxygen, water vapor, etc. At noon or midday, the Sun high in the sky, and light doesn’t have that far to travel to make it to our eyes.

So not all of the blue light is scattered, and that’s why the sky is blue, but at sunset, the light has to travel ten times more atmosphere. We see the long wavelengths of light, and all of the short wavelengths of blues are scattered out. That’s why we see the yellow, the orange, and the red colors.

During the autumn equinox, day and night are equal. Then, each day gets shorter as the sun’s rays approach the northern hemisphere at a more extreme angle. The southern hemisphere is starting to get the more direct Sun but a more extreme angle in the northern hemisphere. It means that now the light from the Sun has to travel even farther at sunset to make it to our eyes. There’s a better chance that more blue light is getting scattered out during that trip and a better chance that we’ll see a yellow, orange or red sunset.

What causes red sky at night? A good sunset depends on the atmosphere’s high pressure because it squashes all those gas molecules. So you get more blue light scattered and more red light hitting your eye. It’s going to travel overnight eastwards.

Why is it so vivid the colors of fall and winter sunsets are beautiful? During the fall in winter, the weather patterns allow for dryness. It means with clean air, the colors of the spectrum make it through without getting scattered by those particles in the air. You would have seen some pretty spectacular sunrises and sunsets where it looks like the sky is on fire and the colors are extra vivid. How does that happen? The best way is for the air to be fairly clear, for brightly colored sunrises and sunsets.

Experimental explanation by physics: Here in the picture, we have a converging lens or convicts lens L1. A strong source of white light earth, which is kept at the focus of this converging lens. Cardboard, there is a hole in it. So after refraction, we will get the parallel beam of light.

Scattering of light by particles
Scattering of light by particles

The parallel beam of light passes through this transparent glass tank, which contains clear water. And when it falls on the cardboard, only a few rays can pass through this hole because all other parts of this cardboard do not allow the light to pass to the site. So some rays of light pass through this hole.

We have another converging lens, another convex lens, which converts these parallel raids onto the screen. Now take about 200 grams of sodium sulfate, Heibel, and dissolve in about two liters of clean water taken in the tank. Guess, first it was pure water, but now the sodium mixture also fits with water in two liters of clean water. Then we add about one to two milliliters of concentrated sulfuric acid to this water.

Then in about two to three minutes, the fine microscopic sulfur particles start precipitating in the tank. As the sample particles form, we can observe the blue light from the glass tank’s three sides because of scattering. So when the beam of light passes through this glass tank, the fine sulfur particles precipitated in the glass tank scatters the light of short wavelengths such as blue light. Because of that, scattering by the colloidal sulfur particles. We observe the blue light from the three sides of the glass thing from this side and then decide which side is opposed.

So when you observe the color of the transmitter light from the glass’s forward side, it is from this site facing the circular hole. We will observe at first an orange-red color. We will observe bright crimson red color on the screen.

Quiz: Why does the sky red during sunset, sunrise, and night?

a) Due to reflection of light.

b) Refraction of light.

c) Dispersion of light.

d) Scattering of light.

The correct answer is it’s due to the scattering of light.


Van Flandern, T.; K. Pulkkinen. “Low precision formulae for planetary positions.” Astrophysical Journal Supplement Series.
Yu Timofeev & A. V. Vasilʹev. Theoretical Fundamentals of Atmospheric Optics. Cambridge International Science Publishing.
Craig F. Bohren & Eugene Edmund Clothiaux. Fundamentals of Atmospheric Radiation: An Introduction with 400 Problems. Wiley-VCH. p. 427.
David K. Lynch; William Charles Livingston. Color and light in nature. Cambridge University Press.
National Weather Service. “Chapter 3: Radiation and Temperature”. Anchorage, Alaska: NOAA.
Darula, S.; Kittler, R. “General Sky Standard Defining Luminance Distributions.” Proc. Conf. eSim 2002. Montreal.
Nave, C. R. “Red Sunset, Green Flash.” Georgia State University. HyperPhysics.

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