Physics Photo of the Week

February 24, 2012

Frost on Edges
On a cold, clear morning in early December last year, a heavy frost formed on some outdoor hardware.  I was intrigued by the concentration of frost crystals on the edges of the hardware as opposed to the general flat areas.  The corners and edges of the bolt and the inside edge of the hole in the steel strap support a large concentration of frost crystals. 

The reason for this concentration around the sharp edges and points is the fact that frost requires a nucleation site in order to form.  In order for water vapor to condense into a liquid or solid, it requires a small object, or a "sharp" edge of a larger object in order to form a nucleation site.  This is a complicated surface effect in which the electric fields at the surfaces become more pronounced in regions of pointed objects that help guide the bipolar water molecules into the solid state.  In the formation of frost, the water vapor condenses directly from the vapor state into a solid - bypassing the liquid state.  This is called sublimation - a change of phase from vapor to solid or vice versa - skipping the liquid state.  Ordinary snow is sublimated water vapor.

The picture at the right shows another close-up of another piece of hardware photographed on the same day.  The frost has formed predominantly on the edges of the square cross-sectioned steel plant hanger.

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

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