Green sky before a tornado is the cloud showing off, not the tornado

A green sky is one of the most misunderstood storm signals. The funnel itself is invisible air; the colour belongs to the cloud deck above it, which is acting as a giant filter for sunlight. ^[1]

What makes the sky green?

Thunderclouds that produce hail and heavy rain are extremely deep, sometimes stretching more than 10 kilometres from base to top. Inside, the water and ice content is so high that sunlight passing through the cloud is strongly scattered and absorbed. The result is that the light reaching the ground is shifted toward the green part of the spectrum, especially when the sun is low and shining nearly horizontally through the storm. ^[1][2]

The effect is strongest in the late afternoon and early evening, when the sun is low and its light travels a long slanting path through the storm. That is also the time of day when severe thunderstorms are most common, which is why green skies and tornado warnings so often appear together. ^[2]

Why the tornado gets no credit

A tornado is a violently rotating column of air. Air is transparent, so a tornado has no colour of its own; what you see is dust, debris, condensation and the dark background behind it. The green tint is a property of the cloud above, not of the funnel. A storm can have a green sky and never produce a tornado, and a tornado can occur under a sky that is not green at all. ^[1][3]

• Green sky is a sign of a very water- and ice-rich thunderstorm, not a direct tornado signal. ^[1]
• The colour comes from sunlight filtered through the cloud, so it depends on the sun's angle as much as the storm. ^[2]
• Tornadoes are colourless; visible funnels are made of condensation, dust and debris, not green light. ^[3]

How to read a green sky

Meteorologists treat a green sky as a hint that a storm is unusually intense and may be capable of large hail and strong winds. It is not a forecast of a tornado by itself. The reliable tornado cues are radar signatures, wall clouds, rotating lowering, a debris cloud at the surface and official warnings. ^[3][4]

So the next time a storm turns the sky green, credit the cloud, not the funnel. The tornado is just passing through a light show it did not create. ^[1]