Physics Photo of the Week

January 26, 2007

Frost Crystals


This common phenomenon on cold, clear winter nights is commonly called "hoar frost".  The temperature is below freezing and the dew point, or condensation point for water vapor in the air is also below freezing.  Water vapor in the air condenses directly as ice on surfaces rather than condensing as liquid.  This process is also called sublimation.  Notice how the tiny crystals of ice are hexagonal in shape.  The angles between the crystal edges are perfect 120 degrees.  The crystals also tend to form on one another, not in isolation.  Once a crystal starts to form on a splinter, dust, or small protrusion in the surface, the edges of the crystal act as nucleation sites for more crystals to form.

The scale of the photo is about 1 cm wide.  It was made by holding a fairly strong magnifier lens up to the camera lens to take the picture.  This is analogous to looking through the magnifier with your eye to see the small details.   The photo below right is the frosty railing where the close-up for the picture at left was taken.

When the ice crystals form in the air as opposed to surfaces, the crystals are "floating" or falling in the air at small speeds.  Because the free-falling crystals are un-impeded by the surface of the object they are attached to, they often form perfectly symmetrical snowflakes with intricate branches.  The free-floating crystals or snowflakes still require a nucleation site to form.  That nucleation site for snow flakes is often a microscopic speck of dust, volcanic ash, or pollution particle.

Watch this page for more frost photos where the frost crystals take different shapes.  A very informative discussion of frost, snow, and ice is maintained by California Institute of Technology professor Dr. Kenneth Libbrecht at snowcrystals.com




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.


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