Did you know that.....(Virginia had a 253 mph wind)?

Did you know that a 253 mph (220 knots or 113 m/s) wind was observed over Virginia on Monday February 5th - at 6:00 A.M. CST (7:00 A.M. EST)?

Now you’re wondering - you didn’t hear about it in the news - no one was talking about a storm or tornado over Virginia.

Well, the southwest-west 253 mph (220 knots or 113 m/s) wind was observed at roughly 33,000 feet (10 km) above the ground (jet stream level) at the Wallops Island upper-air, weather balloon launch site the National Weather Service maintains in eastern Virginia.

The graphic below, from the Storm Prediction Center (SPC) in Norman, OK, shows the jet stream winds at the roughly 33,000 foot (10 kn) level at 6:00 A.M. CST (7:00 A.M. EST).  Note the wind speed values over Virginia.

Weather Map of Jet Stream Nov 5 2007

A weather balloon, with a radio transmitter and instrument package, was launched Monday morning at Wallops Island.  As the balloon ascending into the free atmosphere, it was pushed along by the strong winds aloft.  Radio-tracking of the balloon allowed meteorologists to calculate the wind speed and direction at different elevations above the ground.  In addition, the temperature and dewpoint of the air was measured by the instrument package.

Now you’re really wondering - how did that happen and where did the energy come from?

Well, the frigid air mass over the northeastern part of the U.S. from February 2nd through February 5th set up a very large temperature contrast between New York and South Carolina, where it is typically warmer.  Over South Carolina, the temperature at roughly 18,000 feet (5.5 km) above the ground was -16 C (+3.2 F) Monday morning, while at Buffalo, NY, it was -43 C (-45.4 F).  This huge temperature contrast/gradient was maximized over - you guessed it - Virginia.  In fact, the tight temperature gradient was found between 5,000 and 33,000 feet (1.5 km and 10 km) above the ground!

Physically, the greater the temperature contrast between cold and warm air masses, the greater the amount of potential energy available for the generation of strong winds.  It is the temperature contrast that drives our weather.  Weather is the result of the atmosphere trying to even-out the large temperature contrasts that exist in the atmosphere - as the atmosphere attempts to balance things, the result is weather as we know it.  We owe the large temperature contrasts to uneven heating of the Earth’s surface going from land to oceans, and going from the equator to the poles.

So just how does a large temperature contrast generate strong winds - we thougth you might be wondering?

Here’s a simple explanation -  refer to the graphic below.  As you know, the air gets thinner as we go higher into the atmosphere.  In the graphic , cold air over new York is found on the left side, while warmer air in South Carolina is found on the right side - we are essentially looking at a vertical cross-section of the atmosphere.  The thin black lines represent lines of equal air pressure in the atmosphere, each labeled with P, P-1, P-2, and so on.  An air pressure of P-2 is less than an air pressure of P.   The large blue arrow indicates that "up" is toward the top of the diagram.  Note that the lines of equal air pressure slope downward toward the colder air found toward the north, and the distance between the pressure lines decreases as one goes toward colder air.  This is because cold air is heavy and dense - with its individual air bubbles/parcels packed together more tightly, whereas warmer air toward the south has its in individual air bubbles/parcels packed less densely (warm air is lighter).  In the graphic we have exaggerated the slope of the air pressure lines in order to illustrate our concept.  In reality, the air pressure lines don’t have such an extreme slop, but nonetheless, they do have a slight slope.

Now, if we could magically position ourselves on that thick, black horizontal line, we would find that the air pressure on this line is actually higher over South Carolina (a value between the P and P-1 values), and lower over New York (a value between the P-2 and P-3 values).  Consequently, with our plane of reference restricted to that thick, black, horizontal line, we find lower pressure (the big, black "L") to the north and higher pressure (the big, black "H") to the south.  This pressure difference on the horizontal means that air bubbles/parcels found on our horizontal line will be "pushed" from high pressure to low pressure (much like a ball rolling down hill).  The "pushing" force is referred to as a "horizontal pressure gradient force," which is indicated as a large, red arrow pointing from high pressure to low pressure.  So, our air bubble/parcel is pushed toward the north due to the horizontal pressure gradient force

As our air bubble/air parcel moves horizontally toward the north it will eventually track toward the east because in the Northern Hemisphere the Coriolis force deflects moving objects to the right.  The Coriolis force is the result of our Earth spinning on its axis, and it is exerted on all moving objects such as air bubbles/parcels.  Eventually, the horizontal pressure gradient force (PGF) acting on the air bubble/parcel (pusing it northward) is balanced by the Coriolis force (CF)which pushes it toward the right.  Refer to the graphic below which shows the deflection and eventual generation of a wind blowing toward the east (our westerly jet stream).  In the Southern Hemisphere, where everything is opposite, the Coriolis force deflects air bubbles/parcels to the left.

Generation of Westerly Winds in Jet Stream

So now we have our air bubble/parcel moving to the east at the jet stream level - so how do we accelerate it to the high wind speeds we observe in the jet stream?  The accelerating force is that pressure gradient force we described above.  This force pushes our air bubbles/parcels toward lower pressure, while at the same time the Coriolis force deflects them to the right.   Getting back to our diagram above, the greater the temperature contrast - the greater the slope of our air pressure lines - the greater the air pressure differences on the horizontal - the greater the pressure gradient force that is generated - the greater the acceleration of the air bubbles/parcels - the greater the wind speed in the jet stream!

Besides the large temperature contrast between South Carolina and New York, there was another factor for the high jet stream winds.  Two separate jet streams merged together into one over Virginia Monday morning.  The polar jet stream came in from the northwest (from Montana to Missouri/Illinois) while the sub-tropical jet stream came in from the southwest - from over the western Gulf of Mexico.  When wind currrent merge only one thing can happen with the wind speeds - they must increase/accelerate.  This is known as the Bernoulli theorem in fluid dynamics.  Basically, when moving fluids are forced to go through a smaller opening the speed of the fluid increases.  It’s the same thing with our atmosphere, which is a fluid.  The jet stream winds increase their speed when two different currents come together.

On a related wind-note - the highest wind gust measured at the ground level in the United States was 231 mph (201 knots or 103 m/s) at the observatory on top of Mt. Washington, NH, at an elevation of 6,288 feet (1.92 km).   This location also has an average annual wind speed of about 35 mph (about 30 knts or 15.6 m/s), and once had a 24-hour wind average wind speed of 128 mph (111 knots or 57 m/s) and a monthly average wind speed of 70 mph (61 knots or 31 m/s)!

Going overseas, there is an observation site on the coast of Antarctica that once averaged 44 mph (38.2 knots or 19.7 m/s) winds for an entire year!   These strong winds, known as katabatic winds, are related to the drainage of very cold, dense air from the interior portions of the continent down to the coastal region. 



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