Why does the sky turn green before a tornado?

The green comes from a color-mixing accident between the sun and the storm. Severe thunderstorms tend to fire late in the day, when the sun sits low and its light arrives already tinted red-gold. The storm itself is a towering tank of water — droplets and hail packed miles deep — and all that water scatters and passes blue light better than red. Red-tinted light goes in, the water leans it blue, and what leaks out the underside of the cloud lands somewhere in between: an eerie, luminous green.

The two ingredients

Neither part is exotic on its own. A low sun reddens the light the same way it does at any sunset. And deep water looks blue everywhere — oceans, glaciers, thick clouds. The trick is stacking them: shine reddened light through a water-heavy storm and the surviving color shifts toward green. Computer models back this up — the right cloud thickness plus the right droplet sizes can push the light emerging under the storm into green territory.

Yes, it's really green

For decades people argued the green was an illusion — grass, trees, or hail glinting back onto the cloud base. In 1995, researchers chasing storms across Texas and Oklahoma pointed a spectrophotometer — a handheld meter that reads the exact color of light — at storm after storm, recording hundreds of spectra. Several severe storms measured dominantly green, and the color held whether the ground below was a green wheat field or freshly plowed red-brown dirt. The green is manufactured inside the storm, not reflected from below.

What green actually tells you

A green sky means the storm above you is enormous and loaded with water — the kind that can drop large hail or, sometimes, a tornado. But the same measurements found green hailstorms, gray hailstorms, and tornadoes from skies with no green at all. Treat green as a severity flag, not a tornado forecast: it says take shelter, not take cover from a funnel specifically.