Physics Photo of the Week

December 2, 2011

Ominous Thunderhead - Photo by Gretchen Whipple
Dr. Gretchen Whipple photographed this developing thunderstorm at dusk in late August  from Bedford, VA a couple of years ago.

Thunderstorms often appear in the early evening during the summer months.  They are fed by the rising warm moist air on hot and humid days.  The air cools as it ascends, and clouds form due the the temperature falling below the dew point.  Paradoxically, the condensation of air into cloud releases more thermal energy from the latent heat of fusion.  The extra heat gives the condensing cloud even more buoyancy.  This positive feedback causes the cloud to rise and tower to very high altitudes.  Usually the thunderclouds reach the height of the "tropopause" - the elevation where the ambient temperature suddenly increases instead of decreasing with increasing altitude.  As a result of the high altitude inversion, the cloud no longer has positive buoyancy above the inversion layer and the cloud spreads out - giving the common anvil shaped top.

The time of day - close to sunset - accentuates the details of this cloud - accenting the details through the shadows.  Thanks go to Dr. Gretchen Whipple of the Mathematics Department for this interesting photo.

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 

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: