Why ice hexagonal

This causes the individual crystals to rotate. Generally, the c-axes of the crystals rotate towards an axis of compression and away from an axis of extension. The effect of this is that deep down in the ice sheet the crystals are no longer randomly oriented but have a preferred direction, which depends on the flow history. Thus the flow history of the ice can be found from investigation of the crystal orientation at different depths.

The crystal orientation is determined by studying thin sections of the ice. A thin section is a slab of ice approx. When this slab of ice is placed between two crossed polarization filters, the individual ice crystals can be seen. The colour of the crystal depends on its orientation. Thin sections showing the crystal structure at a depth of a few hundred metres left and from the middle of the ice sheet right. In the top of the ice sheet the crystals have random orientation.

This is seen as the crystals of the thin section to the left having many different colours. Deeper down, the deformation of the ice has lead to the crystals having a preferred direction. Therefore, most of the crystals in the thin section to right have similar colours — blue. The size of the individual crystals also changes with depth. In the Greenland ice sheet these crystals are between 1 mm and 10 cm in diameter.

In the top layers the crystals are generally small, but with time the smallest crystals are 'eaten up' by larger neighbouring crystals causing the size of the crystals to increase with depth.

Close to bedrock the crystals can grow very big because the geothermal heat released from the bedrock increases the growth rate of the crystals. The crystal size also depends on the impurity content of the ice. When there is a high impurity content the crystals tend to be smaller because the impurities inhibit the growth of the crystals. Read more about impurities and crystal sizes here. Read more about how crystal size distribution and orientation are determined from analysis of thin sections.

The plot at right shows the phase diagram of water click on the image for an expanded version. The triple point of water -- when ice, water, and water vapor can coexist -- is at a temperature of 0. Water is the only substance which we commonly experience near its triple point in everyday life. Equilibrium Vapor Pressure of Ice and Water. The plot at right shows the equilibrium water vapor pressure of ice and water as a function of temperature, over the range of interest for snow crystal growth [1].

The vapor pressure is well described by the Clausius-Clapeyron relation, and a fit to the data yields the approximations:. Natural Snowflakes. Designer Snowflakes. Frost Crystals. Snowflake Physics. Snow Activities. Snowflake Touring. Raza, D. Michaelides, B. Slater, Phys. Engel, B. Monserrat, R. Needs, Phys. X , 5 , Casassa, M. Calatayud, K. Doll, C. Minot, C.

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