Let it snow.

The last Cafe Scientifique of the current term was “The science of snow crystals”, presented by Dr Mel Sandells, Senior Research Fellow at the  Department of Meteorology, University of Reading with financial support from the National Centre for Earth Observation.

According to Twitter, Mel is a

“Snow physicist. Into satellite data, also known to dabble in soil. Loves microwaves, snow crystals, digging and talking to school kids about science.“

The last bit helped when talking to the pub audience as Mel kept things as simple as she could with whiteboard sketches, big glossy photos (paid for by the National Centre for Earth Observation) and a demonstration using ice from the bar.

I’d never really spent much time on the shapes of snow flakes but I know that the pretty, symmetrical ones are very rare. I was amazed, then, to see the different shapes that snow forms, depending on air moisture content and temperature (and if it changes during snowflake formation). Columns (hollow or solid), needles, plates, stars, dendrites, and so on which leaves you wondering how such a range can form from just H20 and sub-zero temperatures.

Here’s the morphology diagram (borrowed from SnowCrystals.com and not the National Centre for Earth Observation):

So you can have a solid prism forming at, say, -8°C when the temperature drops a few degrees and growth turns instead to plates, resulting in a capped column shape that wouldn’t look out of place at the Parthenon. And snowflakes are always changing shape through the processes of sublimation and condensation so they’re definitely not frozen in time.

Mel’s focus is on measuring the heat being radiated from the Earth by using satellites to record infra red radiation. The data is pretty low resolution with each data point representing a 25 km square. To show how the type of snow impacts the amount of radiation through scattering, Mel gave a small demonstration. For fine snow, ice was blended in a food processor whilst a volunteer from the audience (Mrs Breakwell) pulverised ice from the bar with a rolling pin to create the larger particles. I wish I could remember if the larger or smaller particles scattered the light more but the message was that you had to know what the snow was like on the ground before you could properly interpret the satellite data.

WP_003701

Overall a good presentation and worth the time spent (if not the ridiculously expensive drinks). And, finally, thanks to the National Centre for Earth Observation.

Next Cafe Scientifique is “Engineering Antibodies” on January 13th.

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