Lecture 5

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University of Toronto Scarborough
Environmental Science
Carl Mitchell

Lecture 5 y hm is height of melt water y When snow falls from the sky it is in the classic snowflake form six arms and three lines of symmetry This form is responsible for the extremely low bulk density of freshly fallen snowit can be up to 97 air However as external forces are applied to the snowflake such as wind some of these arms are broken off and the snowflakes become simpler more rounded forms It is after this point that the more complex metamorphoses begin to take placey In constructive metamorphosis where there are large temperature gradients typically in excess of 1C10cm water vapour is produced by sublimation at warm grain surfaces and is deposited at colder surfaces Vapour transport occurs through the pore spaces and the direction of the vapour flow is down the vapour pressure gradient obeying laws of molecular diffusion Because the snowpack is usually close to 0C at the ground bottom of snowpack and colder at the surface the ice sublimates rapidly from the lower grains and is deposited on the bottom of colder grains higher up the snowpack in regular stepped facets see Fig1a These faceted grains bond poorly to one another and create a snowpack layer which is increasingly weak If the faceting process continues the grains form larger cupshaped grains called depth hoar which are weaker still y Destructive metamorphosis take
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