Physics Photo of the Week
October
8,
2010
Crepuscular Rays
Sunrise
on
October
5, 2010 was a treat with very prominent crepuscular rays - the
shadows formed by the clouds low on the horizon. The shadows from
the low clouds are
visible on the air molecules in the sky. To put it another way,
the sunlight penetrating the air is scattered by the air molecules (as
well as by dust and haze).
Crepuscular means "twilight" from the Latin word for twilight: crepusculum. A number of
animals are called "crepuscular" in that they are active during
twilight such as deer and firefly beetles. This is in contrast to
diurnal (daylight active) or nocturnal (nighttime active)
animals. Thus crepuscular rays are literally
"twilight rays".
Compare this image with previous Physics Photo of the Week images for
crepuscular rays (May
7,
2010) and (December
16,
2006). In today's image the Sun had just appeared at the
horizon (the
converging point of the shadows behind the clouds), and the sunrise sky
color is much
redder than the previous images. This is predominantly on account
of the Sun being much lower in the sky in this week's image. The
sunlight must travel through much more atmosphere. The blue light
has been preferentially scattered out of the direct path of sunlight
leaving the red light to penetrate.
On rare occasions we can look opposite the Sun and see the
crepuscular shadows trailing off in the opposite direction (see PPOW
for
November
10, 2006 for images contributed by Alissa Whalen and
Paul O'Malley). Those crepuscular shadows looking away from the
Sun are called "anticrepuscular rays".
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.
Click
here
to
see
the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
