The visible light spectrum covers colors from violet to red. Each color corresponds to a different wavelength, with green sitting roughly in the middle. Blue and violet have shorter wavelengths. Light rays change direction when they hit particles — a process known as scattering. The sky looks blue because air molecules scatter shorter wavelengths more effectively.
Clouds are made of water drops and ice crystals that scatter light from the sun in all directions. It is the multitude of drops and crystals that make a cloud look white during the day. Sometimes only a small amount of light escapes out the bottom, and so cloud bottoms often appear grayish.
At sunrise and sunset, clouds can appear orange or red. With the sun lower in the sky, the sunlight passes through a significantly longer atmospheric path, resulting in the removal of blue wavelengths before reaching the cloud. Only the longer wavelengths remain visible, accounting for the beautiful orange and red hues observed during sunsets.
Intense thunderstorms sometimes have a green tint. These storm clouds are packed with massive amounts of water and ice and often occur late in the day. The reduced presence of the short wavelength blue portion of the spectrum results from its being scattered out of the column during its long path through the atmosphere. Liquid water has a slight preference for absorbing the oranges and red colors. In the presence of so much liquid water in a thunderstorm cloud, the sligh absorption of red and orange is just enough to filter out some o those longer wavelengths. The middle visible wavelengths, the greens, are then able to dominate and become visible.
While a green sky often suggests severe weather, it does no guarantee you’ll encounter tornadoes or larger hail. The green colo is a visual indicator that the storm has the potential to be severe.
Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.
