ROANOKE, Va. (WFXR) — Are you a weather fan or a meteorological expert? See if you can correctly guess this WFXR Weather Trivia question!

Fill in the Blank: A green sky is typically a sign of _________ ahead.

It all has to do with optics. To understand green skies, you’ll need to know why we generally see blue skies and red sunsets.

The white light from the Sun is made up of all of the colors of the rainbow. When sunlight reaches the Earth’s atmosphere, it is scattered in all directions by the gases and particles in the air. The blue light that makes up part of the sunlight is scattered more than the other colors because blue light has shorter, smaller wavelengths. This is why we see a blue sky when the Sun is closest to us.

During sunsets, we see yellow and red colors for the opposite reason. Red and yellow light is scattered the least as they have longer wavelengths. Sunlight during sunrise/sunset is passing through more of the atmosphere so more red and yellow light passes through.

In order for the skies to appear green, it has to be a certain time of day and weather conditions have to be just right! Usually, green skies can be seen when severe thunderstorms are approaching late in the day.

For the clouds to appear green, the thunderstorm will have to occur around sunset when the sky already appears red and/or yellow. Normally, the light in a cloud is blue due to scattering by water and ice particles within the cloud. The blue objects (water/ice) in the thunderstorm cloud will be illuminated with the sunset’s red/yellow light, making the sky appear green.

According to the National Weather Service in North Platte, it would take “a tremendous amount” of water within the cloud to make this green color. This phenomenon usually occurs with tall and deep thunderstorm clouds capable of producing large hail and/or tornadoes.