RIME, AND REASONYou may be familiar with the common six-sided dendritic snowflake shape, but do you

RIME, AND REASONYou may be familiar with the common six-sided dendritic snowflake shape, but do you know about other ice crystal shapes and how they form? Ice crystals take on a hexagonal shape due to the internal structure of the water molecules. The weak hydrogen bonds of solid water molecules results in a 6-fold rotational symmetry (360°/6, or 60°). However, ice crystals forming in different temperatures and areas with different moisture content take on various hexagonal shapes, including plates, columns, needles, and stars. Snow crystals tend to form simpler shapes when humidity is low and more complex shapes as humidity increases. As snow crystals are transported through a cloud, they move through areas with different temperature and humidity levels. Since no two ice crystals follow the exact same trajectory, nor contain the exact same number of water molecules, it is EXTREMELY unlikely that any two snowflakes will be identical.This photograph of a single capped column snowflake was taken with a Low Temperature Scanning Electron Microscope (LT-SEM). This snowflake began its journey in an area of a cloud that favored columns (-5°C to -15°C, or -30°C and below), then was transported to an area more favorable to plate growth which resulted in caps at each end. As the snowflake fell through the atmosphere, it came into contact with extremely tiny (about 10 μm) supercooled liquid water droplets. These tiny water droplets froze to the surface of the ice crystal, giving it a fuzzy appearance, called riming. The LT-SEM can resolve these tiny snowflake features that would not normally be visible using a light microscope.So, how are these snowflakes captured and prepared for the LT-SEM? The snow is collected using cooled copper plates that contain a methyl cellulose solution. The plates are quickly dumped into liquid nitrogen which cools them to around -196C and allows for safe transport. Finally, they are coated with platinum to make them electrically conductive for the LT-SEM imaging.-AmyFor more LT-SEM snowflake images:http://emu.arsusda.gov/snowsite/default.htmlSnow Crystal Classification:http://tinyurl.com/d8pk3t5 http://tinyurl.com/c9lqe6fAdditional References:http://www.its.caltech.edu/~atomic/publist/rpp5_4_R03.pdf http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm http://www.storyofsnow.com/blog1.php/2011/03/15/how-the-crystal-got-its-sixImage credit: Electron and Confocal Microscopy Laboratory, Agricultural Research Service, U. S. Department of Agriculture -- source link

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