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Physics Photo of the Week

April 15, 2005


Lenticular cloud


A lenticular cloud is relatively rare.  It is the lens-like stack of clouds in the center of the above photo.  Notice the contrasting appearance of the lenticular cloud with the puffy cumulous cloud in the lower center.  The lenticular cloud is formed in the wake of winds blowing over mountains.  The wind was coming out of the northwest over the Great Craggy Mountains near Asheville, NC on October 31, 2004.  After the wind passes over the mountains, the air oscillates between higher and lower elevations.  As the wind ascends in altitude, it cools below dew and cloud particles form.  The lenticular cloud is relatively stationary because the wind waves are stationary, similar to the stationary waves in a rapidly flowing river downstream of a submerged obstacle.  For more discussion of lenticular clouds and another example, please see the link for Physics Photo of the Week for April 9, 2004

The above photoLarge file - 600 kB - please wait... is one shot of a time lapse series shown at right.  Digital cameras and computer image manipulation make it very easy for anyone to make time lapse sequences.  Place the camera on a tripod aimed at the object in question, and press the shutter release every 10 or 20 sec.  The images can be combined in a number of ways - one being to assemble the images into an animated "GIF" image using tools in StarOffice.  The images were snapped every 20 sec and played back at 0.5 sec per frame.  This is a 40x speed-up rate.

Notice the lenticular cloud appears rather stationary compared to the rapidly travelling cumulous clouds.  The lenticular cloud remained in this position for considerably longer than the 4 minute time period this sequence represents.  The cloud re-appeared and faded several times throughout a roughly 15 minute photo session.  The lenticular cloud forms in the wake of air moving over the Great Craggy Mountains.  An analogy may be seen in the ripples in a stream downstream of a submerged rock.  The ripples stay in one place relative to the rock, but the water continues to move downstream.  The lenticular cloud forms in the wake of the mountains from the air rising, then falling again - like the ripples in the stream.  At the high altitude peaks, the air is cooler and the cloud forms because the air temperature falls below the dewpoint.  Further downwind, the air dips back to lower altitudes, where it is warmer, and the cloud particles evaporate.  All photos by Donald Collins.


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.

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.