Weather 101 - What's a derecho? - Local news, weather, sports Savannah | WSAV On Your Side

Weather 101 - What's a derecho?

A downburst and how it forms (Credit: National Weather Service) A downburst and how it forms (Credit: National Weather Service)

Good morning... it's Tuesday... and Labor Day is over with... and its back to work and school. Speaking of school... lets talk weather 101 today!

Did you know that on the evening of June 29th... 2012... a dark bow-shaped cloud raced toward Fort Wayne... Indiana? Probably not I'm guess... well with that ominous cloud came powerful winds reaching 91 miles per hour!

The storm traveled rapidly... and held together. It made it all the way to the Atlantic Ocean by the next morning... leaving a massive trail of destruction in its wake.

Looking at the pictures in the wake of the storm... you might think that a hurricane or a tornado was responsible. But this was a different kind of storm altogether... it was a derecho.

Derechos are fast-moving bands of thunderstorms with destructive winds.

Unlike hurricanes and tornados... these winds follow straight lines (in fact, derecho means straight in Spanish).

It all has to do with something called a downburst. When the wet air in a thunderstorm meets the drier air surrounding it... the water in the air evaporates. When water evaporates... it cools the air around it. Since the cool air is denser... it rapidly sinks to the ground and creates strong winds.

The downburst can actually suck more dry air into the storm... making even stronger downbursts or clusters of downbursts. Derechos occur when the right conditions for downbursts occur over a wide area.

For a storm to be classified as a derecho, the whole area of the storm must have winds of at least 58 miles per hour and it must produce a swath of damage that is at least 240 miles long.

As the storm grows in size... it forms what are known as bow echoes... large bow-shaped packs of thunderstorms that race forward in one direction. These bow echoes form because the downbursts are stronger in the center of the storm. Stronger downbursts mean faster winds. The faster winds race ahead of the storm... creating a bow. They are impressive to see both on the ground and from the sky.

Derechos are most common in the Midwestern United States but are still fairly rare. You might see a derecho about once a year there. They can occasionally be found all the way up into the Northeast.

That's it for today... have a great start to your school and work week. I am scheduled off tomorrow through Friday... so I will be back next Monday with another blog.

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