Physics Photo of the Week

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 photo 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.