Physics Photo of the Week

March 2, 2007

Variations in Frost Crystals


December 9, 2006

January 3, 2006
Notice drastic variations in the basic shapes of the frost crystals on the different days reflecting the different weather conditions.  The crystals on the left from December 9, 2006 (also featured on PPOW for January 27, 2007) are basically hexagonal plates contrasted with the long needle-like crystals observed on January 3, 2007.  Notice also that the needle-like crystals are clustered along some of the wood grain in the wood grain and cracks of the wooden surface where the frost was formed.  The needles most likely are hexagonal in cross section.  Both formations consist of hexagonal ice crystals.  The major difference is that some frost forming conditions favor crystal growth in the basal direction to form hexagonal plates, whereas other conditions favor the growth perpendicular to the basal directions to form long hexagonal needles.  A closer view of the needle crystals is shown at right below. 

Although it is difficult to tell in these photos, the needles are believed to be hexagonal in cross section although some of them look rectangular. 
The longest needles are estimated to be about 3-4 mm long.
  On even closer inspection, it appears that the needles are hollow. 

Why do the frost crystals sometimes form as flat hexagonal plates, while other times they form as hexagonal "pencils"?  The true answer is elusive to physicists.  An excellent article is found on Kenneth Libbrecht's website (snowcrystals.com) and in a recent article he has published in American Scientist Volume 95, pp. 52-59 (January-February 2007).  In this article about the formation and shapes of snow flakes, it is noted that just below freezing (between 0 deg C and about -3 deg C, ice crystals forming directly from freezing water vapor (sublimation) are mostly plate-like.  At lower temperatures (-3 deg C and -10 deg C), the sublimation of water vapor tends to form long hexagonal prisms.  Then again, between -10 deg C and -22 deg C, the morphology switches back to flat hexagonal plates.  These temperatures and the resulting crystal shapes matches the results for the frost presented here.  The temperature of the December 9, 2006 when the hoar frost formed the plates was 9 deg F (-13 deg C) consistent with Libbrecht's range of -10 deg C to -22 deg C for plate formation.  The January 3 temperature, when the hoar frost formed in long needles, was considerably warmer at 19 deg F (-7 deg C) consistent with the columns or needles that Libbrecht reports.  The physical reasons for the different morphologies with different formation temperatures eludes a good physical explanation.  Some of the suggestions involve instabilities at the edges and corners of the crystals that either favor needle-like or plate-like growth and strongly dependent on temperatures.

The temperatures for the different days of frost were obtained from the Warren Wilson Farm Weatherpage.

We will try to photograph some snow flake crystals in the near future, but the weather has been highly uncooperative.  Any contributions from readers would be welcome.




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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