A derecho storm is really a group of severe thunderstorms. (NASA)

A derecho storm is really a group of severe thunderstorms. (NASA)

In a derecho, warm air (red arrows) rises. Moisture in the air evaporates, cooling the air, which becomes a downburst. Downbursts (blue arrows) blow straight down to the ground and then outwards (gray line). (Krieg Barrie)

In a derecho, warm air (red arrows) rises. Moisture in the air evaporates, cooling the air, which becomes a downburst. Downbursts (blue arrows) blow straight down to the ground and then outwards (gray line). (Krieg Barrie)

A derecho storm front moves toward Sarpy County, Nebraska.

A derecho storm front moves toward Sarpy County, Nebraska.

This painting by John Steuart Curry is called The Line Storm. He lived in Kansas, one of the areas where derechos are more common.

This painting by John Steuart Curry is called The Line Storm. He lived in Kansas, one of the areas where derechos are more common.

A radar image shows a derecho in Tennessee. (NOAA)

A radar image shows a derecho in Tennessee. (NOAA)

Dangerous Derechos

Posted: January 1, 2021

A derecho storm is not just one storm. It’s a group of severe thunderstorms with destructive winds.

Tornados spin. But derechos travel in fairly straight lines.

What’s one way that derechos are like tornados? They can cause great damage. Storms must leave a path of damage at least 240 miles long to be called a derecho. Derecho wind gusts blow at least 58 miles per hour. But some derechos blow up to 130 miles per hour. That’s about twice as fast as a cheetah can run!

You might see long, low “shelf clouds” form in front of a derecho. The front edge looks like it’s moving upwards. The bottom of the cloud looks stormy.

What makes derechos so powerful? When the moisture in a thunderstorm meets dryer air, the water evaporates. That cools the air. The cooled air rapidly sinks to the ground as a strong wind called a downburst. Downburst winds blow straight down until they hit the ground. Then the wind blows outwards in all directions.

Downbursts can pull more dry air into the storm. That makes stronger downbursts or even clusters of downbursts. All that strong wind makes the storm very dangerous.

The thunderstorms in a derecho move together in a curved line called a bow echo. The line curves because the downbursts are stronger in the center of the storm. Those downbursts make the center move faster than the edges.

Most derechos happen in warmer weather, usually in May, June, July, and August. But don’t worry. Derechos are fairly rare. Typically, only one to three derechos happen in the United States per year. And if one forms, your meteorologist will warn of high winds, severe weather, or thunderstorms.

If a derecho is near you, find shelter inside. Being outside during a derecho, or any severe storm, is dangerous. Like tornados, the strong winds can bash houses, toss trees, and damage crops.