The Midwest and the Plains states don’t get hurricanes. They get Derechos — sprawling thunderstorm complexes that can travel hundreds of miles and cover multiple states with the impact of a 100-mile-wide tornado.
On Tuesday, a unique weather phenomenon spread out over South Dakota — Parts of South Dakota and Iowa, as well as Nebraska, Minnesota and Illinois, faced a derecho. It even turned the skies green in some areas. Skies in Sioux Falls, South Dakota turned a wicked shade of green as a powerful storm pushed through. Likely a combination of how the colours hidden in sunlight interacted with rain and ice in the clouds.
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. Photos shared on social media documented the eerie phenomenon, with cloud-darkened skies whipping up a nearly photosynthetic hue.
A National Weather Service (NWS) meteorologist said on Twitter that thunderstorm clouds can glow green when the red light of the sun at the end of the day interacts with the water or ice content in the storm. These types of storms can be indicative of hail, the forecaster added, which the Sioux Falls area was hit with on Tuesday.
The NWS declared the storm a derecho – which is a large and continuous event with wind speeds over 58 miles per hour (93 km/h). Over 30,000 people were without power on Tuesday night in the Sioux Falls area, Winds gusted as high as 96 mph in Huron, S.D., the same location that faced 90 mph wind gusts during an equally severe derecho back on May 12. Tuesday’s derecho also brought a gust to 91 mph in Agar, S.D., and 99 mph in Miner, S.D. In Dewey County, S.D., softball-sized hail accompanied winds gusting to 84 mph; the National Weather Service received reports of broken windows, and a machine shed and grain bins destroyed.
The Weather Service received scores of damage reports from South Dakota to Illinois from the violent complex of storms. The winds knocked down power lines and trees, some falling onto homes and vehicles. Tens of thousands of utility customers lost power.
The science behind this phenomenon:
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.
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. 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, as per the report 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.
This was a progressive derecho — or one that rides along a temperature boundary. Progressive Derechos are common in summertime patterns dominated by a high-pressure heat dome. In this case, the heat dome was anchored over the Mississippi Valley, and thunderstorm complexes known as “ridge runners” rode up and around it.
What is a Derecho?
A derecho, according to the US’s National Weather Service is “a widespread, long-lived, straight-line windstorm” that is associated with a “band of rapidly moving showers or thunderstorms”. The name comes from the Spanish word ‘la Derecha which means ‘straight’. Straight-line storms are those in which thunderstorm winds have no rotation, unlike a tornado. These storms travel hundreds of miles and cover a vast area.
Being a warm-weather phenomenon, a derecho generally – not always – occurs during summertime beginning in May, with most hitting in June and July. However, they are a rare occurrence as compared to other storm systems like tornadoes or hurricanes.
For a storm to be classified as a derecho it must have wind gusts of at least 93 km per hour; wind damage swath extending more than 400 km. According to University of Oklahama’s School of Meteorology, the time gap between successive wind damage events should not be more than three hours.
