Derecho turns the sky green in the US

Published: 16th Jul, 2022

Context

States of Nebraska, Minnesota and Illinois in the US were hit by a storm system called a ‘Derecho’.

As the storm rolled in, winds gusting at around 140 km per hour, snapped power lines and knocked down trees.

About

Derecho, according to US's National Weather Service "a widespread, long-lasting, direct storm" associated with "a fast-moving rain belt or thunderstorm".
The word comes from the Spanish word ‘la derecha’ meaning ‘straight’. Straight storms are those where thunderstorms do not change unlike a hurricane.
These storms travel hundreds of miles and occupy a large area.
As it is a warmer climate, Derecho usually - not always - occurs during the summer from May, and hits hard in June and July.
However, they are rare compared to other storm systems such as hurricanes or hurricanes.

Classification

For a storm to be classified as a derecho it must have wind gusts of at least 93 km per hour. The 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.

Types of Derecho

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

What happens in during Derecho?

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

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