Physics Photo of the Week

September 14, 2007

Life of a Thunderhead
Thunderstorms begin as a large cumulus cloud created by thermal updrafts on a hot, humid day.  Within about 15 minutes of this photograph this cloud had developed a flattened top typical of thunderstorms as well as lightning and thunder.

On a typical sunny day, the low level air rises due to heat of the sun.  As the air rises, the expansion due to lower pressure at higher altitudes cools the air.  Soon the dew point is reached and the water vapor in the air condenses into a cloud.  The condensation of water vapor into cloud droplets releases more heat to feed the rising air.  Usually, the cumulus cloud reaches a modest height, then either evaporates or dissipates by the windshear at higher altitudes.  See the PPOW for May 4, 2007



On August 23, 2007 at about 6:00 pm - the date of this photo - there was sufficient water vapor in the air and a relative lack of wind shear that the cloud had enough energy to reach high altitudes where the water drops began freezing into ice and the top of the cloud levels out into the characteristic anvil top as shown in the photo at right.
Photo 2: The anvil beginning to form
Photo 3: The well-formed anvil
Soon the anvil spread out at about 20,000 - 30,000 ft altitude.  Above this altitude the ambient air is warmer than lower altitudes - an inversion - and the cloud no longer has any buoyancy.  The inversion, however, is not warm enough to melt the ice at the top of the cloud - the temperature is about -2
0 deg C.

Thunder could definitely be heard from the cloud at this point.  There were no other clouds visible.  Precipitation can be seen underneath the anvil and on the outside perimeter of the cloud.


This cloud ran out of "fuel".  Apparently, the heating of the lower air diminished (the sunlight was blocked by the cloud) or the moisture had all traveled to the top.  The main body of the cloud seems to be gone - leaving the isolated anvil top.  Curtains of precipitation can be seen below the anvil.  Notice also that the top of the cloud is no longer sharply defined but fuzzy as the convection is no longer feeding new cloud material.  This image (Photo 4) was taken about 30 minutes after the first photo.
Photo Photo 4: The anvil beginning to dissipate
Photo 5: Almost gone
In another 30 minutes the anvil had almost completely dissipated as well as moved further East.  Remnants of the anvil can be seen as an isolated cirrus cloud in the right center of Photo 5. 

More thunderheads in the area did develop that evening.  About 10:30 pm considerable lightning and wind from larger storm did hit the area.


At right is an animation sequence of the developing thunderstorm.  Notice that the cumulus cloud reaches a great height, then spreads out into the anvil top as the updraft meets the upper atomosphere inversion layer.  The animation sequence was created by taking a new photograph every 10 seconds for a duration of about 8 minutes.  The playback is compresed into 3 seconds - a speed-up by a factor of 160 times.

Large file (2.8 MB) - have patiencePhoto 6: Animation of the Thunderstorm development







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.


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