We’ve all seen our share of sunrises, but few have paused to contemplate just why exactly the sky seems to explode with color as the sun rises up to and over the horizon. Pure sunlight is white in color, but contains a spectrum of colors from violet to red. When the sunlight interacts with particles in the atmosphere which are much smaller than the wavelength of visible light, an optical phenomenon known as Rayleigh scattering takes place. During the process of Rayleigh scattering, light is scattered in various directions. The colors with shorter wavelengths like violet, green, and blue are scattered more strongly than orange and red which have longer wavelengths.
Thanks to this effect, the Sun typically appears yellow when seen from the earth, since the shorter wavelengths are scattered into the surrounding sky. During the sunrise and sunset however, when the sun is low on the horizon, the light passes through more air. The additional atmosphere the light must travel through means that more molecules are present to scatter the violet and blue light, but the red, orange, and yellow colors with their longer wavelengths are able to reach your eyes. In fact, when the sun is low in the sky, the Rayleigh scattering effect is increased, removing virtually all blue light from the path to your eyes.
Once the Rayleigh scattering removes the blues and violets from the direct rays, the reddened sunlight which remains is then effected by Mie scattering, during which the sunlight is scattered by larger particles suspended in the air, such as cloud droplets or air pollution. The light effected by Mie scattering lights up the horizon red and orange – without the scattering, the sky along the horizon will have only a dull red appearance, with the majority of the sky appearing blue and occasionally green.
Red sky at night, sailor’s delight.
Red sky at morning, sailor take warning
This well known rhyme might be easily dismissed as superstition by those who have never heard of Mie scattering, but while the navigators of the past may have lacked the modern scientific knowledge we have today, they got this one right. Caused by larger particles in the atmosphere like cloud droplets, Mie scattering helps to produce the vibrant red glow in the sky we sometimes see at sunrise and sunset. Storm systems tend to move eastward, due to the west to east rotation of the earth. A red sunrise can thus be an indication of an approaching storm, arriving from the west, while a red sunset can indicate the the storm is to the east and moving away from the viewer.
The next time you see a brilliant sunrise or sunset, it might just seem even better now that you know why it looks so beautiful.