What is a downburst…and what about wind shear?

Downbursts Introduced

Before examining the formation and consequences of a downburst, it’s important to define wind shear.  Meteorologists use the term wind shear to describe a rapid change in either wind speed or wind direction over a short period of time.   

A downburst is a type of weather phenomenon that accompanies strong to severe thunderstorms.  The birth of a downburst occurs as air within a thunderstorm is rapidly cooled by evaporating raindrops.  As that “pocket” of air is cooled, it becomes denser than the air in its surrounding environment.  Very heavy rain falling in the thunderstorm enhances the downward motion of that air, and it eventually may rush towards the ground (thus the term “downburst”).

As that pocket of air reaches the ground, it spreads out in all directions (“outflow”), causing potentially damaging straight-line winds at the surface.  The strongest wind gust typically occurs in the direction of the storm’s movement.  The leading edge of the outflow is called the storm’s gust front.

downburst1

 

Damage Caused by Downbursts

Downbursts have the potential to produce significant damage at or near the surface.  In fact, the straight-line winds of downbursts have been recorded to reach over 150 miles per hour. This exceeds the wind speeds produced by some tornadoes!    

Speaking of tornadoes, it’s often easy to mistake a downburst for a tornado.  One way storm spotters can determine which went through a particular area is by surveying the damage.  Since downbursts produce straight-lined winds that flow outward from the storm, debris often lies parallel to the wind’s outflow.  For example, a cornfield that has been flattened in which each stalk is facing the same direction would likely be the cause of a downburst.  On the other hand, winds in a tornado actually flow inward and rotate.  In some tornado situations, lines of debris may converge toward the axis of the tornado path.  Consequently the debris from tornadic winds often lie at various angles.   

Below is a table depicting the typical range of wind speeds seen in downbursts (50-140 mph) and the damage caused by those wind speeds.

 
A Historical Downburst in Wisconsin

One of the more significant downburst events to take place in the Badger State occurred on July 4th, 1977.  Thousands of acres of forests were completely flattened by winds estimated to have approached 115 m.p.h.  

For more information on this particular event, please click here.

 Downbursts and Aviation

In addition to producing significant wind damage at the surface, downbursts pose a high threat to airborne flights.  The formation of vertical wind shear makes for very difficult take-offs and landings.  For example, in the graphic below an airplane is beginning its landing.  After entering the initial gust front, the pilot must deal with flying into a strong headwind.  This increases wind speed over the aircraft, increasing lift and causing the plane to rise above its ideal path flight.  In order to compensate for this, the pilot must lower the throttle to decrease the plane’s speed.  Upon passing through the rainshaft, winds shift dramatically and the pilot must now account for a strong tailwind.  This decreases the flow of wind over the wing, reducing lift.  Even with the pilot applying full throttle in order to increase lift, it’s often not enough and the plane may crash short of the runway.

 

 

The Life Cycle of a Downburst

 
As seen above, the formation of a downburst begins with an initial downdraft that is typically less than one mile in diameter.  A downburst usually makes contact with the ground within 5-7 minutes of development.  One cannot really “see” this happening, but it can be felt at the surface as winds pick up and temperatures drop (again, this is called the gust front).  This is usually followed by a period of rain and possibly hail.  After about 10 minutes, the downburst peaks in wind shear intensity.  These maximum wind gusts usually last between two and four minutes.  Finally, the downburst dissipates.  Although the size of the downburst peaks around 2.5 miles in diameter at this stage, the winds are significantly weaker.

 

Wind Shear Revisited

In order to illustrate the importance of wind shear in tornadic development, the F3 Stoughton tornado of August 18, 2005, will be examined.  Below are a couple graphics depicting the estimated vertical wind profile in the Stoughton, WI area at 23Z (6:00 P.M.)...

 

Notice that winds at the surface were between 5 and 10 miles per hour, and they were coming from the south.  5000 feet above the surface, winds increased to 41 mph.  Additionally, the wind direction at that height shifted from southerly to southwesterly.  At 10,000 feet, winds increased to 46 mph and were coming from the west, south-west.  By the time 35,000 feet was reached, winds had increased to 81 mph from the west.

This classic case of tornado formation exhibited two types of wind shear: directional wind shear and speed wind shear.  Both are essential to have in place for tornado development.  Directional wind shear was present because as elevation increased, the wind direction veered (changed direction in a clock-wise motion).  Speed wind shear was also present because the wind speed increased with height.  This created a rolling effect in the atmosphere, a key component to mesocyclone rotation development within the thunderstorm (which eventually led to a tornado). 

Here is a map showing the path of that F3 tornado.  It touched down just southeast of Fitchburg and took a mainly easterly route, ending 2.2 miles north of Busseyville in Jefferson County.  The total path length was 20 miles and the tornado was on the ground for about 53 minutes. 

Kyle Meier-NWS Milwaukee/Sullivan Student Volunteer

Portions of information courtesy of NWS Columbia, SC and NWS Green Bay, WI

 



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