What is a derecho, the rare windstorm turning sky green in the US?

In an unusual weather event on Tuesday, states of Nebraska, Minnesota and Illinois in the United States were hit by a storm system called a derecho. Tuesday's derecho produced a swath of damage over 600 miles long from northwest South Dakota to northern Iowa. Winds blowing at around 140 km per hour snapped power lines and knocked down trees. 

As the storm hit, it turned the skies green. Images were shared on social media which showed how surreal this natural phenomenon was. The storm not just left behind considerable rainfall, hail and wind but also an ominous green sky that left residents amazed. 

Read | Unusual weather phenomenon turned sky green in US's South Dakota, looks like scene from sci-fi movie

About 30,000 homes and businesses were without power in South Dakota. This was the second time South Dakota was affected by a derecho in less than two months. Heavy rain also lead to local flash flooding in some spots. So what is a derecho, lets find out here.

What is a derecho?

Derechos are rare storms that can be just as powerful and destructive like hurricanes or tornadoes. They are primarily seen in late spring and summer in the central and eastern United States. Derechos produce walls of strong wind that streak across the landscape, leaving hundreds of miles of damage in their wake. 

Derecho name comes from the Spanish word 'la derecha' which means straight. In straight-line storm winds, there is no rotation unlike a tornado. These storms travel hundreds of miles and cover a vast area. Derecho is a warm-weather phenomenon and generally occurs during summertime.

Only those storms are classified as a derecho which have wind gusts of at least 93 km per hour and 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.

Why did the sky turn green?

Derechos result in a green sky due to light interacting with the huge amount of water they hold. A report in the Washington Post suggest that the big raindrops and hail scatter away all but the blue wavelengths due to which primarily blue light penetrates below the storm cloud.

This blue then combines with the red and yellow of the afternoon or the evening sun to produce green, the report said. Derechos fall into three categories - progressive, serial and hybrid.