A green or blue-green sky can appear beneath some severe thunderstorms, often when the Sun is low and the storm contains abundant water or hail. The exact optical explanation is not settled, and the colour does not reliably predict a tornado. NOAA says scientists do not agree on why the sky may look green. Use official warnings and radar—not sky colour—to make safety decisions.
The colour depends on light and storm structure
Light reaching an observer has travelled through the storm and its precipitation. The amount and type of water, ice, cloud depth and angle of sunlight can alter which wavelengths are scattered or transmitted. One common explanation combines a deep, water-rich cloud with low-angle reddish sunlight, producing a greenish appearance, but no single account explains every observation.
A green sky is not a green flash emitted by a tornado. The storm’s particles and the lighting geometry change how the cloud appears to an observer.
Why scientists are cautious
NOAA’s National Severe Storms Laboratory says scientists do not agree on the cause of green-looking skies, even though the colour is often reported around severe weather. [1] Different cloud depths, hail, rain and sunlight can create different viewing conditions, making simple one-line explanations unreliable.
Anecdotes about a green sky and a tornado happening together do not establish that the colour predicts tornado formation. A storm can be dangerous without a green appearance, and a greenish view is not proof that a tornado is present.
How to stay safe
Treat a severe thunderstorm watch or warning as the signal to act. Move indoors, stay away from windows and follow local emergency guidance. Do not wait for a particular sky colour, and do not go outside to photograph a storm.
The safest conclusion is modest: green skies are an interesting visual clue to unusual storm lighting, but they are not a forecast tool.
Sources and evidence
- NOAA NSSL — Severe Weather 101: Hail FAQ ↗
States that scientists do not agree on why skies sometimes appear green.
- NOAA — Thunderstorm hazards and tornadoes ↗
Official storm hazard and safety information.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Original explanatory article prepared from linked scientific and institutional sources; scope and uncertainty are stated in the text.