In areas like the Central United States that get lots of tornadoes,
there's a common piece of celestial fortune-telling.
People say that green skies mean a tornado is on its way.
Research has shown that it isn't quite as simple as that,
but scientists have found that if you see a green sky,
you should probably go inside.
As far as most scientists can tell,
the green skies around powerful thunderstorms are usually a combination of red sunsets and water droplets.
Like we've talked about before,
daytime skies are blue because bluer, shorter wavelengths of visible light
tends to bounce off air molecules better than redder, longer wavelength light.
So the blue light gets bounced all over the sky
and looks like it's coming from everywhere.
Meanwhile, around sunset, sunlight travels through so much atmosphere
that just about all the blue is bounced away from the horizon,
leaving all those picturesque reds and oranges behind,
but that quick summary hides something important.
Sunsets might look exclusively red and orange,
but there's still some green and even blue light hidden in there,
just far less than the other colors.
But for us to notice the residual green light,
it needs to hit something that reflects green light much better than red.
That's where water comes in.
Big, tall, threatening storm clouds are made of water droplets,
and even though water is best at reflecting blue light,
it can still reflect green pretty well, much better than reds and oranges,
so under just the right conditions, the water in and around a cloud
can bounce the green light hidden in the sunset right into our eyes making the sky look green.
This explains why most of these eerie skies are reported late in the day
when there's not much blue light around to dominate.
Of course, green skies can happen in the middle of the day, too.
Enough blue light just has to get bounced elsewhere
by the right combination of air molecules above the clouds and the water within them.
Still, none of the conditions that can turn skies green are unique to the clouds that spawn tornadoes.
They're often associated with these twisters,
because the intense storms that produce green-tinted skies can also produce tornadoes,
but they don't always.
Sometimes they just lead to lots of rain, and maybe some hail.
So if you see huge storm clouds rolling in and a green tint in the sky,
it doesn't necessarily mean there's about to be a tornado,
but it does mean you should probably head indoors.
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