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