The sky takes on orange and red hues at sunset because sunlight must pass through a much thicker layer of atmosphere; this longer path scatters the shorter blue wavelengths, allowing the longer red and orange colors to dominate.
How scattering works
- Rayleigh scattering: Air molecules scatter blue light far more efficiently than red light, so as the sun moves lower toward the horizon, blue light is removed from the direct line of sight.
- Optical path length: With the sun low on the horizon, light travels through several kilometers of atmospheric density, which increases the scattering effect.
Additional particles in the air can intensify these hues:
- Dust and soot: Larger particles favor Mie scattering, which is more efficient at scattering red light, making sunsets appear deeper and more dramatic.
- Air pollution and aerosols: Cities with high levels of pollutants often have brighter, more saturated sunsets due to additional scattering.
- Volcanic ash: A major eruption can inject sulfate particles into the stratosphere, producing intense, spectacular colors around the world.
- Humidity and clouds: Humidity can soften the colors, while clouds can act as a canvas that reflects and amplifies the orange/red tones, creating stunning displays.
In short, the characteristic orange and red sky at sunset is the result of the physical combination of a longer atmospheric path (which removes blue light) and the presence of particles that can enhance the remaining red/orange light.
