Physics Photo of the Week

September 20, 2013

Breaking Wave in the Clouds
That ominous hook-shaped cloud over the Craggy Mountains at Warren Wilson College resembles an ocean wave breaking onto a beach.  Time lapse photography shows that the motion of the cloud exactly resembles the dynamics of a breaking wave at the beach.  Watch the animation below.

Typical water waves driven by the wind depend on the fact that the surface of the water moves much more slowly than the wind.  The surface of the water thus experiences a large "shear" force.  Whenever two different fluids (different densities, viscosities, etc) experience such a shearing motion, the denser fluid tends to build up in piles, thus a wave crest is generated.  In the atmosphere air near the Earth's surface is denser than air at higher altitudes.  In the time-lapse video clip, a wind shear can be seen in the clouds at different elevations.  Notice that the wave crest of this photo builds up, the top is moving forward faster than the base, thus the higher density part of the air - that which is visible as a cloud - becomes unstable and "crashes" onto the cloud below.  An equally good analogy would be to compare this "breaking" wave to a white-cap on lake waves when the wind (shearing) is exceptionally strong.

The drawings below shows the effect.  Both panels show two layers of different density fluids (or air above water).  The arrows show the velocities of each layer.  The different in the velocities indicates a large velocity shear.  The left panel represents the "starting" of the shear.  The shearing along the interface soon results in more dense material building up as waves.  The middle right wave crest in the right panel shows a crest that is "white capping" or spilling over and "breaking".


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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