Why sunsets

Within the visible range of light, red light waves are scattered the least by atmospheric gas molecules. So at sunrise and sunset, when the sunlight travels a long path through the atmosphere to reach our eyes, the blue light has been mostly removed, leaving mostly red and yellow light remaining.

The result is that the sunlight takes on an orange or red cast, which we can see reflected from clouds or other objects as a colorful sunset or sunrise.

Large particles of pollution or dust scatter light in a way that changes much less for different colors. The result is that a dusty or polluted sky is usually more grayish white than blue. Similarly, cloud droplets typically 10 millionths to millionths of a meter are much larger than visible light waves, so they scatter light without much color variation. This is why light scattered by clouds takes on the same color as the incoming light.

That is, unless you like hazy sunsets. Corfidi works in storm prediction, but has a lifelong fascination with the science of sunsets. He first got interested in meteorology, he says, because of sunsets.

Every day brings a new sunset, and he wanted to know why. Of course. Beauty is subjective. Sunset colors are created by a phenomenon called Rayleigh scattering. Sunlight contains all the colors of the rainbow. But not all the colors reach the ground in the same concentration.

Nitrogen and oxygen molecules in our atmosphere act as little mirrors for blue and violet light, in particular.

That means not as much blue or violet light reaches the ground. At sunset, light has to travel through a greater distance of atmosphere to reach our eyes — so even more blue light, and even some green and yellow light, gets filtered out. But why are they seemingly more saturated during the colder months?

But the explanation for why low humidity produces better sunsets is a bit indirect. These can be chemicals naturally produced by trees, they can be from the wind kicking up dirt and dust into the atmosphere, or they can be from man-made pollution sources. When these aerosols attract water vapor, they swell. And in fall and winter, weather patterns usually result in a clearer lower atmosphere, creating a better chance for a vivid sunset.

This causes the blue light that is seen during the daytime to be scattered away from our eyes, allowing more orange and red light to pass through.

Science tells us that the essence of a beautiful sunset is in the cloud layer — specifically the clouds at the upper and lower levels. Molecules and small particles in the atmosphere change the direction of light rays, causing them to scatter. Scattering affects the color of light coming from the sky, but the details are determined by the wavelength of the light and the size of the particle. The short-wavelength blue and violet are scattered by molecules in the air much more than other colors of the spectrum.

This is why blue and violet light reaches our eyes from all directions on a clear day.