This week, the skies above Nebraska, Minnesota, and Illinois changed to green like a Harry Potter movie. 

This was not a special effect, nor were there any dark wizards or murderous spellcasters. It was a storm system called a derecho.

With winds of around 140km/h, the storm ripped power lines and uprooted trees as it moved in. 

According to the Washington Post, storm chasers who have been through many of these storms claim to have never seen it before.

What is a derecho, exactly?

According to the US National Weather Service, a derecho is "a long-lasting, widespread windstorm that causes severe weather conditions." 

It can also be associated with "bands of swiftly moving thunderstorms or showers."

Spanish derecha means straight. Unlike a tornado where winds rotate, a derecho is a straight-line thunderstorm without any rotation in its wind winds. These storms can travel hundreds of miles and cover large areas.

Derechos usually occur in the summertime, with the frequency increasing from May to June to May. Even in cold weather, there have been cases of derecho. 

A derecho, however, is rarer than other storm systems such as hurricanes and tornadoes.

It is considered a derecho if a storm has wind gusts exceeding 93 km/h and wind damage swathes greater than 400 km. 

According to the University of Oklahama's School of Meteorology, the time between wind damage events should not exceed three hours.

There are three types of Derechos: progressive, serial, and hybrid. A progressive derecho refers to a line of thunderstorms traveling hundreds of miles on a narrow path. It is a summer phenomenon.

The serial derecho has a long, wide, continuous squall line that sweeps across large areas. This usually happens in spring and fall.

Hybrids have both serial and progressive Derechos.

The Super Derecho, which occurred on May 8, 2009, was one of America's most intense and unusual Derechos. It swept across the US from Kansas to Kentucky at speeds up to 170 km/hr.

Why did the sky turn green?

According to the reports, severe thunderstorms can turn the sky green from light interfacing with large amounts of water.

According to a Washington Post report, raindrops can scatter any blue wavelengths except blue because only blue light penetrates the storm cloud.

The report stated that this blue is combined with the red-yellow from the afternoon or evening sun to create green.