KIDSGAZETTE

The Iowa derecho: How those winds became so strong

Radar From the Advanced Weather Interactive Processing System (AWIPS) display inside KDVN during the derecho in Iowa. Se
Radar From the Advanced Weather Interactive Processing System (AWIPS) display inside KDVN during the derecho in Iowa. Severe thunderstorm warnings are lined in yellow, with tornado warnings in red. (National Weather Service)

Two weeks ago, the weather turned scary.

Some winds were so strong, they snapped trees in half. Some heavier trees fell, their deep roots pulling the ground up with them. Debris flew everywhere, crashing into houses, apartments and cars. Rain seemed to come down horizontally. The sky looked green.

The storm, called a derecho, caught many people by surprise. How did it happen?

A derecho — which means “straight” in Spanish — starts with “something called a downburst,” according to the National Oceanic and Atmospheric Association. The air in a normal thunderstorm is wet, and when it meets drier air outside of the storm, evaporation happens.

As evaporation happens, it makes the air around it cooler. That cool air sinks — and to us, that sinking feels like strong winds. The downburst can pull even more dry air into a storm, and that creates more downbursts, and the dangerous cycle continues.

To be a true derecho, a storm’s winds must be at least 58 miles per hour. Some of ours were faster than 100 miles per hour. It also needs to cover at least a 240 mile long area. Ours covered 700 miles.

You might be worried about another storm like that happening again. No one can predict the future, but storms of that force are rare.

There might be one once a year here in the Midwest, but our recent storm was uniquely destructive. It’s unlikely to happen twice in the same place.

Comments: (319) 398-8330; molly.duffy@thegazette.com

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