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Derecho turns the sky green in the US

  • Category
    Geography
  • 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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