Ice Crystals
Clues from the cloudsRachel Schwartz April 17, 2009
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Why Study Ice Crystals?
• What controls the shape?– Cloud temperature– Water vapor content
• History• Classification• Measurements
Ukichiro Nakaya - historical classification
• 1611 Kepler
• 1931 Bentley - photographs
• 1954 Nakaya – first systematic classification scheme
Classification
• Sectored Plates
• Stellar Dendrites
• Fernlike Stellar Dendrites
• Bullet Rosettes
• Needles
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Temperature and Supersaturation
• Nucleation and vapor deposition• Supercooled water • At -20 C supersaturation with respect to ice is 21%! ---> rapid growth
Crystal Shape - Temperature and Supersaturation
• From: Basic shape– Hexagonal prism
• To: “No two are alike”– Particle and heat
transport
– Surface diffusion and chemistry
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Rimming and Aggregation
• Rimming– Supercooled water
droplet collides with ice crystal
• Aggregation– Ice crystal collides
with ice crystal
Observing Ice Crystals
• Sampling surface– Cooled black velvet
• Magnification– Microscope
– Magnifying glass
• Patience– Most are irregular!
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Ice crystals in the lab
• To understand growth processes
• Vapor diffusion chamber
• Electric needles• Time lapse - movie
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Clues from the clouds
• Careful examination can tell us about– In cloud temperatures
• Riming --> faster with higher liquid water content
• Aggregation --> ice crystal concentrations
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References:Peterson, T.C. , Yeh . J , Cotton, W.R. Manual for Snowflake Observation, Identification and Replication, Colorado State University Kenneth G. Libbrecht, Caltech SnowCrystals.com Kenneth G. Libbrecht, The physics of snow crystals, Reports on Progress in Physics 2005