Photo of the Week
in Frost Crystals
December 9, 2006
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
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
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
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.
Photo of the
published weekly during the academic year on Fridays by the Warren
Wilson College Physics
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explanation. Atmospheric phenomena are especially welcome.
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