The appearance of vibrant colors, particularly shades of pink and purple, that illuminate the sky during the brief windows of sunrise and sunset is a phenomenon governed by atmospheric optics. This stunning display results from the way sunlight, which appears white, interacts with the gases and tiny solid or liquid components that make up Earth’s atmosphere. The specific colors we see are directly tied to the characteristics of the light itself and the distance that light must travel before reaching an observer.
The Mechanism of Light Scattering
Sunlight is a form of electromagnetic radiation composed of a spectrum of colors, each corresponding to a different wavelength. When this light enters our atmosphere, it encounters countless gas molecules, primarily nitrogen and oxygen. The interaction between light and these molecules causes the light to be redirected in various directions, a process known as scattering. Light waves with shorter wavelengths, such as blue and violet, are scattered much more readily by these tiny atmospheric molecules than longer wavelengths, such as red and orange. This disproportionate scattering of short-wavelength light is the reason why the sky appears blue during the day. Conversely, the longer-wavelength red and orange light passes through the atmosphere with relatively little deflection.
The Role of the Sun’s Angle
The phenomenon of colorful skies at the beginning and end of the day depends on the sun’s position relative to the observer. When the sun is high overhead, its light takes a relatively short and direct path through the atmosphere. However, at sunrise or sunset, the sun sits very low on the horizon, forcing its light to travel a significantly extended distance through the atmosphere’s dense layers. This dramatically increased path length amplifies the scattering effect on the shorter wavelengths. Over this long journey, nearly all of the blue and green light is scattered away from the direct line of sight. What ultimately survives this extensive filtering process are the longer, less-scattered wavelengths, leaving the sky dominated by the warm tones of red and orange.
How Atmospheric Particles Create Pink Hues
While the sun’s low angle explains the shift to red and orange, the distinct appearance of pink or purple requires the presence of additional, larger airborne particles. These particles, which include aerosols, dust, smoke, or water droplets, cause a different type of scattering called Mie scattering. Unlike Rayleigh scattering, Mie scattering is not strongly dependent on the light’s wavelength and tends to scatter all colors more uniformly. When the remaining red light from the setting sun encounters these larger particles, the light is scattered in a way that can mix with small amounts of residual blue or violet light scattered from the higher atmosphere. This combination results in the softer, pastel shades of pink and purple. The vividness of these pink skies is often enhanced by atmospheric conditions like humidity or fine dust lofted from wildfires or volcanic eruptions.