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  • Published
    9th Jul, 2022


States of Nebraska, Minnesota and Illinois in the US were hit by a storm system called a derecho. As the storm hit, it turned the skies green.

What is a derecho?

  • Definition- 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”. 
  • Wind Speed- 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. 
  • Occurrence- Being a warm-weather phenomenon, a derecho generally – not always – occurs during summertime beginning May, with most hitting in June and July. However, they are a rare occurrence as compared to other storm systems like tornadoes or hurricanes.

Why did the sky turn green during the derecho that hit US recently?

  • Light interacting with water- Severe thunderstorms result in a ‘green sky’ due to light interacting with the huge amount of water they hold.
  • Scattering of wavelength- A report in the Washington Post said that it is believed that the big raindrops and hail scatter away all but the blue wavelengths due to which primarily blue light penetrates below the storm cloud.
  • Production of green color- This blue then combines with the red-yellow of the afternoon or the evening sun to produce green.
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