Explained: What is a derecho, a storm that turned the sky green in the US?

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 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.

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

Severe thunderstorms result in a ‘green sky’ due to light interacting with the huge amount of water they hold. 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. This blue then combines with the red-yellow of the afternoon or the evening sun to produce green, the report said.

Oh my gosh! Look at how green the sky is ahead of severe storms near Sioux Falls, SD! #sdwx pic.twitter.com/fc7fpN6loL

— Chris Michaels (@WSLS_Michaels) July 5, 2022

Are there different types of derechos?

They fall into three categories – progressive, serial and hybrid. A progressive

derecho is associated with a short line of thunderstorms that may travel for hundreds of miles along a relatively narrow path. It is a summer phenomenon.

A serial derecho, on the other hand, has an extensive squall line – wide and long – sweeping across a large area. It usually occurs during spring or fall.

Thank you for sharing these incredible photos! https://t.co/TEBzIXcIgQ

— NWS Sioux Falls (@NWSSiouxFalls) July 5, 2022

Hybrid ones have the features of both progressive and serial derechos.

Where do derechos usually occur?

They mostly occur across central and eastern parts of the United States.

The May 8, 2009 “Super Derecho” was one of the “most intense and unusual derechos ever observed” in the US as it swept from Kansas to Kentucky with wind speeds reaching up to 170 km/hr.

Derechos have also been documented elsewhere across the world. In 2010, Russia witnessed its first documented derecho. They have also swept through Germany and Finland, and more recently in Bulgaria and Poland.