Context: Recently, the states of Nebraska, Minnesota and Illinois in the US were hit by a storm system called a derecho.

About Derecho

• A derecho 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 in May, with most hitting in June and July.

How To Identify A Derecho

• 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.

• The wind damage swath extends more than 400 km.

• The time gap between successive wind damage events should not be more than three hours.

Different types of Derechos

They fall into three categories –

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

• Hybrid derecho: These ones have the features of both progressive and serial derechos.

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

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

• 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.