Physics Photo of the
Week
December 9, 2011
Moonrise
In
mid August, 2 days before the full moon, I was captivated by
the rising Moon in the south east and the striking illuminated
cumulus cloud. The Sun had not yet set, so the cloud was
still in daylight. The Moon had risen about 2 hours
previous because it was about 2 days before the completely
full phase. Notice the reddish tint to the cloud due to
the long path the sunlight travels in the Earth's atmosphere
when near the horizon. The blues become scattered and
absorbed by the atmosphere and only the longer red wavelengths
survive to illuminate objects. The sky is blue because
the short blue wavelengths are scattered by the atmosphere to
the camera or observer.
In the time-lapse animated sequence at right, we can see that
the cumulus cloud is very dynamic. The cloud consists of
rising warm moist air soon condensing into cloud. In
this cloud, the faster higher level winds shear the upper
parts 
of the cloud laterally - limiting the maximum
altitude reached by the cloud. In the absence of the
wind shear, this cloud could have developed into a
thunderstorm.
The animation spans a time of about 6 minutes compressed to
about 2 seconds. In this time, we can see that the
shadow of the Earth's horizon on the cloud is propagating up
the cloud as the Sun is setting.
Also the path of the Moon is inclined from the vertical and
sloping to the right (southeast). This is due to our
latitude on the Earth. At WWC's latitude of about 35
deg, the rising angle is inclined at 35 degrees from the
vertical. When viewed from the southern hemisphere, the
angle of sunrise and moonrise is inclined to the north (see PPOW
for Sept 23, 2005 and PPOW
for Sept. 13, 2004
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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here
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