Wearing of the Green- Are Green Skies usually associated with Severe Weather? (Photo Credit: Jennifer Shoemake)
Shelf cloud taken from Albuquerque, New Mexico's west side on July 26, 2018.
Across the Great Plains, folklore says that if the sky turns green it’s time to head inside. But, is there truth to this saying? While it is not uncommon for green skies to accompany severe weather, there appears to be no direct correlation between the two.
During the day, the sky appears blue because the shorter-wavelength, bluer end of the light spectrum bounces off of air molecules better than the long wavelength red end of the spectrum. As the sun shifts lower in the sky (and after the peak heating of the day), the spectrum of direct sunlight is shifted from blue wavelengths of the sky to the red, orange, and yellow of the sunset due to the longer trip of the sun’s rays through the atmosphere. A thick cumulonimbus is composed of many water droplets and ice particles, resulting in air molecules scattering and attenuating. Since water reflects blue and green light better than red, there appears to be a green-ish tint to the sky. The same phenomenon is present when you place a glass of water with blue food coloring in front of a glass with blue that will produce the same green tint that the light in the sky transmits.
A study from Pennsylvania State University concluded that the relative contribution of hail to the green color was actually quite small. It isn’t the presence of hail needed to produce a green sky, but the size of the droplets/particles in the cloud may dictate the shade of green (for example, smaller drops may lead to more of a blue-green sky than larger drops which may produce more of a yellow-green color).
These intense thunderstorms that produce green-tinted skies also have the potential to generate large hail, damaging winds, frequent lightning, flash flooding, and even tornadoes, which may make the green color appear threatening. But, this is not always the case. Either way, it is best to head indoors when thunder roars.
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©2020 Meteorologist Sharon Sullivan