Why Are Sunsets Red?

During the day, sunlight enters the atmosphere nearly vertically (at noon) and travels through a relatively thin layer of air. Blue light (shorter wavelength, ~450 nm) is scattered about 5.5 times more than red light (~700 nm) by nitrogen and oxygen molecules (Rayleigh scattering), colouring the sky blue while the Sun appears white-yellow. At sunset, sunlight enters at a shallow angle and must travel through up to 30 times more atmosphere. This extended path means that virtually all blue and violet light is scattered away before reaching your eyes, leaving predominantly red and orange wavelengths. Dust, pollution, and volcanic aerosols in the lower atmosphere enhance the effect by providing additional particles for scattering and absorption of shorter wavelengths. This is why volcanic eruptions (like Pinatubo in 1991) and wildfire smoke produce especially vivid red sunsets worldwide. The same physics explains why the Sun itself appears progressively redder as it approaches the horizon.