Physics Photo of the Week

January 25, 2008

Snow Squall
On 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.

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.