Why Is the Sky Blue?
The sky appears blue because of Rayleigh scattering - air molecules scatter shorter (blue) wavelengths of sunlight more than longer (red) wavelengths. Blue light scatters in all directions across the sky, so when you look anywhere overhead, you see scattered blue light. At sunset, light travels through more atmosphere, scattering away blue and leaving red/orange.
Key Takeaways
- Sunlight contains all colors of the visible spectrum.
- On the Moon (no atmosphere), the sky is black even during the day.
- Mars has a butterscotch-colored sky due to dust particles in its thin atmosphere.
Explanation
Sunlight contains all colors of the visible spectrum. When sunlight enters Earth's atmosphere, it collides with gas molecules (mostly nitrogen and oxygen). These molecules scatter shorter wavelength light (blue and violet) much more effectively than longer wavelengths (red and orange) - about 10 times more.
This scattering, called Rayleigh scattering, sends blue light in all directions. When you look at the sky away from the sun, you see this scattered blue light coming from all parts of the atmosphere. Violet light actually scatters even more, but our eyes are more sensitive to blue, and some violet is absorbed in the upper atmosphere.
At sunrise and sunset, sunlight travels through more atmosphere to reach your eyes. Most blue light gets scattered away before reaching you, leaving the longer red and orange wavelengths to color the sky. Dust and pollution can enhance red sunsets by providing additional scattering particles.
Things to Know
- On the Moon (no atmosphere), the sky is black even during the day.
- Mars has a butterscotch-colored sky due to dust particles in its thin atmosphere.
- Water is actually very slightly blue for similar scattering reasons, not just from reflecting the sky.