Physics Photo of the Week

October 5, 2007

Wind Gusts
Looking down onto Willoughby Lake, Vermont, one can see an array of wind gusts.  The gusts darken the appearance of the water. The stronger wind creates a rougher water surface which reflects the sky differently.  The wind out of the North is blowing generally towards the observer in the photograph.  One can see about 4 separate wind gusts as "fronts" perpendicular to the wind direction.  The gusts are separated by about 300 - 500 meters.

Winds are seldom steady, but fluctuate in windspeed as gusts.  The main reason for the wind gusts rather than a steady wind is due to the difference in windspeed between the surface of the earth and and higher elevations above the earth - a windsheer effect.  Because of the slower wind speedLarge file loading - have patience at the earth's surface (or water surface) the air "piles-up" in bunches.  Periodically the increase in air density of a "bunch" becomes unstable and the bunch spills out creating a turbulent gust front.  The gust fronts propagate in the same direction of the wind flow as one can see from the animation at right. 

The animation was made with a handheld digital camera taking a frame about every 5 seconds.  With the lack of a tripod (on the remote mountaintop) the images had be be aligned then played back with a speed-up factor of about 60 times.  One can see a hiker on the lookout below moving about almost erratically. 

Also notice that the cloud shadows are traveling at about 90 degrees to the wind on the lake to the left of the picture.  Careful inspection will even show the cloud shadows on the water - much fainter than the gust fronts.

Wind gusts are annoying while sailing as the wind appears intermittantly, but gusts can become quite dangerous in aviation.  The physics of gust formation is quite complex.  Not all winds are gusty.  The presence of gusts is apparently associated with the wind shear gradient.

Similar effects can be seen with water flow - especially if a film of water with the proper thickness flows over a relatively smooth surface such as rain water on a street.  See the photo at left.

Other examples of "gusty" flow include highway traffic with "unexplained" traffic jams; astrophysics with the turbulent inflow of material on an accretion disk; and whistling teakettles.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


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