Physics Photo of the Week

January 28, 2011

Needles from the sky
On January 10 this winter - during one of the major snowstorms - it was very cold with a light snow falling.  This snow was unusual for ground-reaching snow in that the snowflakes were tiny needles.  One snowstar is visible in this photo.

A fine dusting of snow lay on the vinyl grill cover, the mesh pattern is aout a 2 mm in size
.  These snow needles are thus about 3 mm long and about 0.1 to 0.2 mm across.  A close-up photo of a cluster of shorter needles is shown below.

These snow needles are not nearly as
spectacular as snow stars (see PPOW for January 22, 2010).  However, these needle-like crystals are important in producing wonderful optical effects in the atmosphere when they form at high elevations in some cirrus clouds.  Because of the hexagonal nature of ice, these snow needles are believed to have hexagonal cross-sections resembling hexagonal wooden pencils.  When they fall they tend to line-up with the long axes horizontal.  Light from the Sun or Moon shining through these crystals creates the spectacular Solar and Lunar 22 degree haloes (see PPOW for December 3, 2010).

To produce the close-up photos without an expensive macro lens, I handhold a fairly powerful magnifier up to the camera lens and take the photo.  It is rather difficult to hold the lens centered on the camera lens without scratching the surface in addition to keeping the optics in focus.  I used a "paparozzi" mode - snapping a continuous stream of images while slowly moving the camera-subject distance to have a better chance of capturing an in-focus image.



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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