I’ve heard this question a few times over the years and just got it again yesterday, so I figured it is probably time to answer it on the blog!
The short answer: Physics! The long answer: It doesn’t always.
Light is cool
When we look up in the middle of a clear day we see the blue sky. The sky is blue because the molecules in the air are good at scattering blue light.
In fact, there is some research that suggests the sky is actually more purple than blue, but our eyes are designed to pick up blue light better, so we see blue. But that’s for a whole’nother post!
Think back to your Science classes in high school and middle school. I’m sure you remember learning about ROYGBIV. The part of the Electromagnetic Spectrum that our eyes can see. We also call this Visible Light.
And the order of ROYGIV is in increasing energy. So R (red) has the longest wavelength and V (violet) has the highest wavelength. It is the main reason why the sky is blue during the day and then redder at sunset.
The light has to pass through more of the atmosphere and is, thus, less energetic because it is plowing through more of the atmosphere.
The same thing happens in a thunderstorm. Just not to the same degree.
The light from the sun is darkened by the clouds, sure, but the light is also scattered by the raindrops – and hailstones.
The raindrops, like all water, have a tendency to refract the light, too. That means the light is now moving at a slightly different angle. You have seen an example of this if you’ve ever looked at a straw in a glass of water.
But not only is it bent, but it is also less energetic. Moving the light, every so slightly, from one energy level down to the next. So violet light turns to blue, blue turns to indigo, and indigo turns to green!
We did it, right?
Light is weird
It turns out that we need another thing to happen, too. We need the sun to be at a bit of an angle behind or adjacent the storm itself, relative to the person looking into the storm.
Because light works in funny ways. It isn’t like painting with water colors.
I turns out that you need a mixture of blue, indigo, yellow and orange light.
And you get the blue and indigo light by the sunlight enter the thunderstorm from the top of the cloud and being bounced around. Then you get the yellow and orange light from the sunlight that may enter the thunderstorm lower down that has already had some of the energy removed from it by traveling through the atmosphere.
The easiest way to accomplish those things is to see the thunderstorm between sunset and about 4 hours before sunset.
Alright! We figure it out, now, right?
Light does this often
Notice I have yet to say anything about the hailstones yet aside from they are also in the cloud with the raindrops. That is because there has yet to be any research to support that only hail turns clouds green.
That’s right!
A paper in 1996, titled, “Green Thunderstorms Observed” by Frank W. Gallagher III, William H. Beasley, and Craig F. Bohren explained that “green thunderstorms exist,” but they noted, “the exact definition of green is visually subjective and is based upon the perception of the observer. We have observed and recorded spectra of thunderstorms that appear green to the human ob-server from both ground level and from the air. We have further demonstrated that the optical properties of thunderstorms do undergo transformations, often with a change in color toward green, during their life-times. Furthermore, we have seen both green and non-green clouds, in the same storm, which almost certainly had hail during the entire observational period. This does not rule out the possibility that the presence of hail may contribute to the green appearance of some thunderstorms, but it does suggest that the presence of hail may not be required in order for a storm to appear to be green. It is possible that the storms observed to be green in these two cases satisfy the requirement of Bohren’s hypothesis (which does not require hail) thunderstorms that appear green to the human observer from both ground level and from the air. We have further demonstrated that the optical properties of thunderstorms do undergo transformations, often with a change in color toward green, during their life-times., but were also so large that hail was produced by them as a byproduct.“
So, hail isn’t what makes clouds green. And the clouds don’t always turn green before it hails.
Instead it is the size of the storm and the angle of the viewer relative to the storm and the Sun. One could argue, that there is a happy coincidence that occurs. Large storms offer more refraction and energy loss. Large storms are also more likely to produce hail.
There have been a handful of scientific research papers published with a few different hypothesis — but one hasn’t been more accepted than the other, meaning it is still up for debate. Some propose it is light behind the storm, others suggest it may be light ahead of the storm.
All hypothesis though, agree that it is based on a relationship between the Sun’s angle, the storm and the viewer of the storm.
The green color has nothing to do with hail.
Don’t be fooled by coincidences
So then why does everyone say “the sky turns green from hail” or “the sky turns green before a tornado” then?
Because it has happened.
Hailstorms and tornadoes often take a few hours to form from singular puffy cumulus clouds. Often it takes hours. And often, in the Plains, the hours it takes are from about 1p through about 5p. And then the storms roll along between 5p and 10p.
And guess what is happening in the Spring an Summer from 5p through 10p? Sunset!
So people look up and see big storms coming, get a Warning from the NWS, and happen to coincidentally be at the correct angle relative to the storm and the Sun and the sky turns green. And then, as a coincidence, that storm produces some hail or even a tornado.
And the journalists go and talk to the people after the storm asking, “what did you see?” And people say, “well before the storm got here, the sky turned all green.”
But it wasn’t because there was a tornado or hail in that storm. It was more about where the person was relative to the Sun and the storm.
Because when the next storm blow through, at the same time and the same place, but is just a few rumbles of thunder and some heavy rain. It may also be green. But because there were no warnings, so no one looked up, there no damage, so no journalists came to ask questions.
And no one ever knew the sky also turned green without any hail or tornadoes!
Well, until now.
We sure could use you in Hattiesburg with all these storms blowing through.