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

Source: The post is based on the article “Explained: What is a derecho, a storm that turned the sky green in the US?” published in The Economic Times on 9th July 2022.

What is the News?

States in the US were hit by a storm system called a Derecho storm. As the storm hit these states, it turned the skies green with many experienced storm chasers claiming to have never witnessed such atmospheric optics.

What is Derecho?

A derecho is a widespread, long-lived, straight-line windstorm that is associated with a band of rapidly moving showers or thunderstorms.

Origin of Name: The name comes from the Spanish word ‘la derecha’ which means ‘straight’. 

Note: 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.

Occurrence Period: Being a warm-weather phenomenon, a derecho generally 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.

Types of Derecho: They fall into three categories – progressive, serial and hybrid. 

– Progressive Derecho: It 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.

– Serial Derecho: It has an extensive squall line – wide and long – sweeping across a large area. It usually occurs during spring or fall.

– Hybrid Derecho: They are the ones that have the features of both progressive and serial derechos.

Where does Derecho occur? ​​They mostly occur across central and eastern parts of the United States. However, Derechos have also been documented elsewhere across the world such as in Germany, and Russia.

Why did the sky turn Green during Derecho in the US?

Severe thunderstorms result in a ‘green sky’ due to light interacting with the huge amount of water they hold. 

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.