1. Ice Crystals Clues from the clouds Rachel Schwartz April 17, 2009

2. Why Study Ice Crystals? What controls the shape? –Cloud temperature –Water vapor content History Classification Measurements

3. Ukichiro Nakaya - historical classification 1611 Kepler 1931 Bentley - photographs 1954 Nakaya –first systematic classification scheme

4. Classification Sectored Plates Stellar Dendrites Fernlike Stellar Dendrites Bullet Rosettes Needles

5. Temperature and Supersaturation Nucleation and vapor deposition Supercooled water At -20 C supersaturation with respect to ice is 21%! ---> rapid growth

6. Crystal Shape - Temperature and Supersaturation From: Basic shape –Hexagonal prism To: “No two are alike” –Particle and heat transport –Surface diffusion and chemistry

7. Rimming and Aggregation Rimming –Supercooled water droplet collides with ice crystal Aggregation –Ice crystal collides with ice crystal

8. Observing Ice Crystals Sampling surface –Cooled black velvet Magnification –Microscope –Magnifying glass Patience –Most are irregular!

9. Ice crystals in the lab To understand growth processes Vapor diffusion chamber Electric needles Time lapse - movie

10. Clues from the clouds Careful examination can tell us about –In cloud temperatures Riming --> faster with higher liquid water content Aggregation --> ice crystal concentrations 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