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 speed 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
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
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
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.
Photo of the
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 firstname.lastname@example.org.