Explainer: What made the skies in the United States turn green?

New Delhi: On Tuesday, the states of the US – Nebraska, Minnesota, and Illinois were hit by a storm system called a derecho. The winds were blowing at around 140 km per hour, which snapped power lines and several trees fell. However, what turns out to be interesting is that as the storm hit, the skies turned green with many experiencing storm chasers saying that they have never such atmospheric optics before, as per Washington Post.

What is derecho?

According to US’s National Weather Service, a derecho is “a widespread, long-lived, straight-line windstorm” that is associated with a “band of rapidly moving showers or thunderstorms.” Derecho comes from the Spanish word ‘la dereche’ which means ‘straight.’ Straight-line storms are regarded as those in which thunderstorm winds do not rotate like a tornado. These storms travel miles after miles and cover vast areas.

Derecho is a warm-weather phenomenon, however, does not always occur during summertime beginning in May, mostly hitting in June and July. These storms are rare occurrences compared to other storms like tornadoes or hurricanes.

To categorise a storm as a derecho, the winds have to blow at a rate of at least 93 km per hour, with winds damage covering an area of more than 400 km. According to the University of Oklahama’s School of Meteorology, the time gap between successive wind damage events should not be more than three hours.

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

Multiple thunderstorms result in a ‘green sky’ owing to light interaction along with a large amount of water they hold. As per the report by the Washington Post, it is believed that the big raindrops and hail disperse away all but the blue wavelengths due to which blue light enters below the storm cloud. The blue light then combines with the red-yellow of the afternoon or the evening sun to produce a green colour.

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

Types of derecho

Derecho falls into three categories – Progressive, serial, and hybrid.

A progressive derecho is related to the short line of a thunderstorm that may travel for hundreds of miles along a comparatively narrow path. This is a summer phenomenon.

A serial derecho has an extensive squall line both wide and long spreading across a large area. This phenomenon happens during spring or fall.

A hybrid derecho is a mixture of both progressive and serial.

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

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

Where so derecho takes place?

Derecho is mostly seen across central and eastern parts of the United States. 

On May 8, 2009, “Super Derecho” was one of the “most intense and unusual derechos ever observed” in the US, covering Kansas to Kentucky with winds blowing up to 170 km/hr.

Derecho also occurred in other parts of the world. In 2010, Russia for the first time witnessed derecho. The winds blew through Germany and Finland and more recently in Bulgaria and Poland.