Why is the sky blue?
The short answer
The sky looks blue because sunlight is scattered in many directions as it passes through Earth's atmosphere.
Blue light is scattered more strongly than red light. That scattered blue light reaches our eyes from many parts of the sky, making the whole sky look blue.
Understand it in 30 seconds
Sunlight looks white, but it is a mixture of many colors, including red, orange, yellow, green, blue, and violet.
Earth's atmosphere is made mostly of nitrogen and oxygen molecules. These molecules are much smaller than the wavelengths of visible light, so they scatter shorter wavelengths more strongly. This effect is called Rayleigh scattering.
Blue light has a shorter wavelength than red light, so air molecules scatter it many times. When we look up, we see not only light that has traveled directly from the Sun, but also blue light that has been scattered toward us by the atmosphere.
Why does the sky look blue instead of violet?
Violet light has an even shorter wavelength than blue light, so it is scattered strongly too. The sky does not usually look violet because several factors work together:
- sunlight contains less violet light than blue light
- human eyes are less sensitive to violet than to blue
- some violet light is absorbed in the upper atmosphere
As a result, blue is the color that stands out most to our eyes.
Why are sunsets red?
When the Sun is low in the sky, its light travels through a longer path in the atmosphere before reaching us.
Along that path, much of the blue light is scattered away. The direct light that remains near the Sun contains relatively more red and orange light, so the sunset can look red or orange.
Dust, smoke, and other particles in the air can make these colors stronger.
Why are clouds white?
The water droplets and ice crystals in clouds are much larger than individual air molecules.
They scatter many visible colors more evenly instead of scattering blue much more strongly than red. The mixture of colors reaches our eyes as white light. Thick clouds can look gray because light is absorbed and scattered many times inside them.
A common misunderstanding
The air is not producing a blue dye or a blue glow by itself. Sunlight is being scattered by the atmosphere, and blue light is more likely to reach our eyes from different directions.
That is also why the part of the sky away from the Sun looks black in space, where there is almost no atmosphere to scatter the light.