Physics Photo of the
Week
September 30, 2011
Pileus cloud
Notice the small wispy
cloud at the top of this large
cumulus cloud. The cumulus cloud is developing into a
small
thunderstorm. Such clouds are formed from strong
thermal
convection. When the vapor-laden air condenses to form
the cloud
(due to adiabatic expansion as the air rises), the large
latent heat is
released giving the cloud more buoyancy and driving the cloud
tops even
higher. Because these clouds contain rapidly rising air
masses,
some of the drier high altitude air that is not part of the
cumulus cloud gets pushed to higher altitudes
ahead of the cloud. The resulting condensation of the
drier air
on top of the cloud forms a wispy cloud of ice crystals on top
of the
cumulus cloud. This small wispy cloud is called pileus after the latin
word for
"skull cap".
The animation sequence shown at right shows the vigorous
upward
movement of the cloud. The pileus "cap" shows only
briefly in a
couple of frames of the animation sequence.
Thunder was heard in this storm cloud during the photo
session, and a
flat "anvil" top was also seen - but soon obliterated by
another storm
approaching from behind the camera. No rain fell on my
location
from either storm.
This animation was taken by setting the interval timer on the
camera to
take a picture once every 10 seconds. The animation is
played
back at 20 frames/second - a speed up factor of 200. The
photographs were taken about one year ago (September 22,
2010).
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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