Ice Crystals

Ice Crystals
Sebastian Hoch &
C. David Whiteman
Atmos 3200/ Geog 3280
Mountain Weather and Climate
Vapor deposition - Bergeron/Findeisen process
Whiteman (2000)
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Saturation vapor pressure for ice is lower than that for water
Air is near saturation for water, but is supersaturated for ice
Ice crystals/snowflakes grow by vapor deposition
Cloud droplets lose mass to evaporation
Sector plate
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Stellar dendrite
Dendritic sector plate
Hollow column
Habits – types of ice crystal shapes
created by vapor deposition
Needle
Snowcrystals.net
Plates
Stellar crystals
Columns
Needles
Spatial dendrites
Capped columns
Irregular particles
Graupel
Sleet
Hail
Depositional Growth
Solid precipitation types
Riming
Refreezing of melted snow
“Big-time” riming
Magono and Lee’s classification of snow crystals
Crystal type vs temperature and supersaturation
Ken Libbrecht's Field Guide to Snowflakes (Voyager Press, 2006)
Ice crystal growth
Ice crystal habits
stellar dendrites
needles
plates
spatial dendrites
hollow columns
sectored plates
rimed crystals
capped columns
oddballs
© Kenneth G. Libbrecht
Unusual snow crystal forms
12-sided snowflakes
triangular snowflakes
twin prisms
bullets
arrowhead crystals
pyramidal crystals
© Kenneth G. Libbrecht
Formvar crystal replicas
Liquid method: Dissolve 1g of polyvinyl
Formvar resin (Resin 15-95E, Monsanto
Chemical Corp., Springfield , MA) in 100
ml ethylene dichloride. Cool the 1%
solution and a glass microscope slide
below 0°C. Wet the glass slide with this
liquid and then let crystals fall into it.
Place in an area protected from the
snow but still at ambient temperature
until the solvent has evaporated (3-5
min). The crystals are encased in a thin
plastic shell. Bring indoors and let water
evaporate through plastic shell, leaving
a clear plastic replica of the snowflake.
Spray method: Use clear acrylic spray
paint from the hardware store
Schaefer and Day 1981
Selection method - as shown
How can we make snow?
What do we need to make
snow?
Source of liquid water,
compressed air, ice
nuclei,
cold air temperatures.
The Dolomites, August 24, 2003
© C. D. Whiteman
Are any two snow crystals alike?
Ice Crystal Optics
Halos at the South Pole
http://www.atoptics.co.uk/halo/spole.htm
Molecular Structure of Ice Crystals
All crystals have
identical interfacial
angles!
Why: they are all
hexagonal prisms
of varying habit:
ranging from long
columns to thin
plates.
Alignment: due to
air resistance they
are often aligned in
a certain way.
http://www.atoptics.co.uk/halo/crhal.htm
Atmospheric Halos
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random alignment - 22˚ halo
plate arcs or halos
column arcs or halos
http://www.atoptics.co.uk
A 22° circular halo surrounds a low
sun. To the left and right are colored
sundogs. Above shines an upper
tangent arc and beneath is a sun
pillar. HaloSim simulation of a fairly
common ice crystal halo display.
© Wes Cowley
Sundogs = parhelia
Sub-sun & sub-parhelion
© 2004 Helen Todd
© 2004 Helen Todd
Sun pillar
HaloSim
http://www.atoptics.co.uk/halo/downld.htm
Hole in cloud
© Brooks Martner
Links and Sources:
http://www.atoptics.co.uk/
http://www.its.caltech.edu/~atomic/snowcrystals/