Physics Photo of the Week
January 19, 2008, the weekend before classes resumed at Warren Wilson,
we had some on and off snow showers, or snow squalls. The photo
doesn't look very spectacular, but notice that the distant mountain,
called "White Oak Flats" directly beyond "Watch Knob", is mostly
obscured by snow.
The picture becomes more interesting when animated by means of
time-lapse photography in the picture below on the right. Frames
in the time-lapse sequence were snapped once every 10 seconds and
played back at 10 per second. That speeds up the motion by a
factor of 100. Two physics effects are demonstrated by these
photos: the direction of motion of the falling snow and the visibililty.
The snow is "falling" nearly
horizontally in the direction of the wind. The wind was blowing
out of the northwest (diretly left of the photos) at a moderate speed
of 5 mi/hr or 2.2 meter/sec (WWC
Farm Weather Site). Snow flakes fall through the air very
slowly due to the large amount of friction per unit mass.
Typically snowflakes fall at the terminal velocity of 0.3
meter/sec. Since the wind speed is about 10 times the falling
rate for snowflakes, the snow cloud trajectory is practically
horizontal. Another consequence of the slow falling rate for snow
flakes is that it takes on the order of an hour for the snow to reach
the ground from a snow-forming cloud that is about 3000 ft above ground
level. It is fascinating to watch the winter storms approaching
in the mountains.
The other affect is the visibility. Each snowflake is about 1 mm
in diameter. In the midst of the snow squall there are about 100
snowflakes per cubic meter of air. The average clear distance
(average distance in the snow squall in a straight line before the line
of sight encounters a snow flake) follows the simple formula: dist =
1/(area x density). Inserting the numbers (1 mm)2 for
the area of a snowflake and 100/m3 for the number density
gives the average distance of about 10 km. White Oak Flats, the
mountain in the distance about 8 km from the viewing site, is barely
visible through the snow squall - indicating its distance is
approximately the average clear distance. This assumes that the
density of snowflakes is uniform over all that distance. As can
clearly be seen, the density of snow is highly variable.
Therefore the assumptions made here are very approximate.
Photo of the
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 firstname.lastname@example.org.
here to see the Physics Photo
the Week Archive.
Observers are invited to submit
digital photos to: