Why are skies turning green in South Dakota?

On Tuesday, a unique weather phenomenon spread out over South Dakota — the sky turned green.

As a storm approached, residents of the Sioux Falls area shared pictures and videos of storm clouds brewing against a glowing lime-green backdrop, casting a spectral pall over the landscape.

The reason? Likely a combination of how the colours hidden in sunlight interacted with rain and ice in the clouds, reported 9News.

As light passes from one substance into another, it can bend and warp in a process called “refraction”.

That’s essentially what’s happening in a prism. White light is made up of every visible colour combined, which all have different wavelengths. When that white light enters the glass of a prism, each colour bends a little differently because of those different wavelengths — separating each colour into its own visible ray of light.

The same kind of warping can happen in rain clouds, too.

As the sun’s light passed through the clouds in South Dakota’s storm, the light bent as it interacted with water. While it’s not completely certain why the sky turned green, one theory behind such skies is that the normal blue colours produced during a storm might mix with a yellower sky to form a green appearance, according to previous reporting at The Weather Channel.

The fact the sky turned green means there was likely a ton of water and ice in those clouds, leading to a ton of refraction, 9News says.

Tuesday’s storm was part of a “derecho”, a kind of thunderstorm characterized by especially high winds. While there isn’t too much evidence yet connecting derechos to the climate crisis, The Washington Post has reported that some scientists are starting to see a potential link between planetary warming and the likelihood of these dangerous storms.

The climate crisis has been implicated in other events where the sky has turned strange colours, too. In 2020, parts of California saw orange skies as wildfires ripped through the state and spewed smoke through the atmosphere.

And other fires have left parts of the world with grey skies as pollution and smoke choked off the light.

Things like thunderstorms and wildfires are expected to get more intense over the coming decades in some parts of the world as the greenhouse gas-driven climate crisis raises temperatures, dries out landscapes and adds more moisture to the atmosphere.