The sun’s red appearance at sunrise and sunset is a captivating natural phenomenon. This color change is due to the intricate interplay between sunlight and Earth’s atmosphere.
The Nature of Sunlight and Atmospheric Scattering
Sunlight, which appears white, is a composite of all colors, each with a different wavelength. When passed through a prism, it separates into violet, blue, green, yellow, orange, and red. Violet and blue light have shorter wavelengths, while orange and red light have longer wavelengths.
Earth’s atmosphere consists of gas molecules, primarily nitrogen and oxygen, which are much smaller than visible light wavelengths. When sunlight encounters these particles, it undergoes scattering, where light waves are redirected.
The scattering of light by particles much smaller than the light’s wavelength is known as Rayleigh scattering. This phenomenon dictates that shorter wavelengths of light, such as blue and violet, are scattered much more efficiently than longer wavelengths, like red and orange light. Consequently, as white sunlight enters the atmosphere, a significant portion of its blue and violet components is dispersed across the sky.
Why the Sun Appears Red at Sunrise and Sunset
During the middle of the day, when the sun is high, its light travels a shorter distance through the atmosphere. Some blue light is scattered, making the sky appear blue, but enough other colors reach us, making the sun appear yellow or white.
As the sun approaches the horizon, its light travels through a significantly greater thickness of the atmosphere. This extended journey means sunlight encounters more gas molecules, scattering an even larger proportion of shorter-wavelength blue and violet light.
With most blue and violet light scattered, longer-wavelength colors dominate the remaining light. Red, orange, and yellow light, less prone to scattering, penetrate the atmosphere more effectively. These colors predominantly reach our eyes, making the sun appear distinctly red or orange.
Beyond Rayleigh Scattering: Other Influences on Color
While Rayleigh scattering is the primary mechanism, other atmospheric components influence the sun’s color. Larger particles like dust, smoke, volcanic ash, pollution, and water droplets are suspended in the lower atmosphere. These particles scatter light differently than gas molecules.
These larger particles engage in Mie scattering, which occurs when particles are comparable in size to light wavelengths. Unlike Rayleigh scattering, Mie scattering is less wavelength-dependent and affects all wavelengths more equally. Dust or smoke can enhance blue light scattering, intensifying the sun’s redness. Volcanic ash can also create vibrant or unusual hues.
Humidity also plays a role; larger water droplets contribute to light scattering and absorption, modifying the sun’s appearance. The presence of these additional atmospheric particles can lead to a wide spectrum of sunset and sunrise colors, from deep reds to fiery oranges and purplish hues.