Why does the sky sometimes turn green before a storm?

The exact cause is debated, but the leading theory involves the interaction of reddish late-afternoon or sunset light with the blue tones of a massive cumulonimbus cloud laden with water and possibly hail. When the golden sunlight enters the deep cloud, the water and ice scatter it in a way that emphasises green wavelengths. Another factor may be the sheer depth of the cloud filtering out other colours. A green sky does not guarantee a tornado, but it is strongly associated with severe thunderstorms that can produce large hail, which requires the very tall, water-heavy clouds that create the green tint.