What’s Up With The Wind?

***Watch WISN 12 News for the latest weather information!***

************Update**************

Here are the peak wind gusts from Tuesday

  • 3 miles SE Pewaukee  55 mph
  • Milwaukee – Mitchell  49
  • West Bend  48
  • Grafton  47
  • Kenosha  47
  • Racine  46
  • Port Washington  45

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Thank you for stopping by the Weather Watch 12 Blog!  The big weather story for Tuesday will be the wind.  A Wind Advisory is in effect for all of southern Wisconsin.  Sustained winds of 20-35 mph can be expected, with wind gusts above 40 mph.

Fall is typically one of the windiest times of the year in Milwaukee.  As the atmosphere tries to remain in balance through the ups and downs of Fall, pressure differences(sometimes quite large) occur.

You have probably seen a surface map marked with H’s and L’s which indicate high and low pressure centers. Surrounding these “highs” and “lows” are lines called isobars.  “Iso” means “equal” and a “bar” is a unit of pressure so an isobar means equal pressure. We connect these areas of equal pressure with a line.  Everywhere along each line is constant pressure.  The closer the isobars are packed together the stronger the pressure gradient is.

Pressure gradient is the difference in pressure between high and low pressure areas. Wind speed is directly proportional to the pressure gradient. This means the strongest winds are in the areas where the pressure gradient is the greatest.

The pressure gradient force is a force that tries to equalize pressure differences.  This is the force that causes high pressure to push air toward low pressure.  Thus, air would flow from high to low pressure if the pressure gradient force was the only force acting on it.

Below is a diagram showing high and low pressure, isobars, and the flow from high pressure to low pressure.

So why does air spiral out from highs and into lows? There is one other force, called Friction, which is the final component to determining the flow of wind. The surface of the earth is rough, and it not only slows the wind down but it also causes the diverging winds from highs and converging winds near lows.

Now that we have looked at the forces and other factors that cause the wind, let’s look at a real world example.  With today’s gusty winds in place, this is a perfect time to look at a surface map.  Below is a surface map from around 7:30 a.m. Tuesday.  I labeled the center of low pressure over eastern Minnesota.  Notice the winds wrapping into the low in a counter-clockwise direction.  This is just like the example above.  Click on the image below to enlarge.

The westerly winds on the back side of the low over Minnesota are also ushering in cooler temperatures to southeast Wisconsin.  Ahead of a cold front early Tuesday morning the temperature in Milwaukee at 2 a.m. was 77 degrees!  Just four hours later behind a cold front the temperature dropped to 64 degrees!

Cooler weather will now take hold through at least Thursday.  Nighttime lows will be around 50 near the lake, with 40s well inland.  And highs will only top out in the 60s!  The official start of Fall is still a couple of weeks away, so enjoy the final days of summer.

In Wednesday’s blog we’ll talk about how the remnants of Tropical Storm Hermine may bring our area rain to start the weekend.

If you have any questions about the wind or the weather…just post your thoughts to the comments section of the blog.

Have a great day!

Jeremy Nelson

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