Why So Windy?

By: Travis Feltner
By: Travis Feltner

It sure has been a windy weekend so far...here's why.

What is wind?  It's simply air in motion.  We can't actually see it, but we know it's there, because we can see the trees sway or a flag flying in it.  We measure wind speed with a device called an anemometer and its direction with a wind vane.  When you hear a forecast for northwest winds of 15 to 20 mph, as we've seen today, that means the winds will be 15 to 20 mph FROM the northwest, not blowing to the northwest.

So, how is wind generated?  On a surface map, we have areas of high pressure marked with H's and areas of low pressure marked with L's.  Around these highs and lows are lines called isobars.  Isobar means equal pressure.  Here's what this may look like.

  Image from the National Weather Service.

The closer the isobar lines are packed together, the stronger the pressure gradient force.  So, what's the pressure gradient force?  It's the difference in pressure between high and low pressure areas.  Where the pressure gradient is greatest, winds will be strongest and vice-versa.  Also, the pressure gradient force is a force that tries to equalize pressure differences, so it causes high pressure to push air toward low pressure like this. 

  Image from the National Weather Service.

Here's what our surface map looked like for November 15, 2008.


You can see the area of high pressure marked by the H well off to our west in Texas and the area of low pressure marked by the L off to the northeast.  Notice the isobars are close together across the Southeast in comparison to say, the plains states.  This means the pressure gradient force is great, so winds will be strong...as they were from the northwest at 15 to 20 mph.  Why were they from the northwest you may ask.  As the colder air behind the cold front to our east filters into the area from the northwest, the winds shift to out of that direction behind the front.  This is known as cold air advection.

Of course, there are a couple other forces that cause wind to move as it does, but this will give you an idea of what to look for on a surface map to know if it's going to be a windy day or not.

-Travis
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