Why is the Sky Blue?

The sky appears blue because of a process called Rayleigh scattering, which occurs when sunlight passes through the Earth’s atmosphere and is scattered by the air molecules: 

• SunlightThe sun emits a spectrum of colors, each with a different wavelength. Blue and violet light have shorter wavelengths, while red light has a longer wavelength. 
• ScatteringWhen sunlight reaches the Earth’s atmosphere, the air molecules scatter the light in all directions. Blue light is scattered more than other colors because of its shorter wavelength. 
• PerceptionOur eyes are more sensitive to blue light, and the sun emits more energy as blue light than violet. 

Here are some other things to consider about the sky’s color:

• HorizonThe sky appears paler near the horizon because the scattered blue light has to pass through more air, and some of it is scattered again. 
• ElevationAt higher elevations, the sky can appear very dark blue or bluish-violet because there are fewer molecules to scatter light. 
• SunsetsAs the sun gets lower in the sky, more blue light is scattered, allowing the reds and yellows to pass through to your eyes. 
• HistoryThe sky was once orange because of the toxic vapors in the atmosphere. Cyanobacteria in the ocean created oxygen through photosynthesis, which slowly changed the sky’s color to blue over time. 

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