Geoscience Reference
In-Depth Information
(1977) suggested a mass exchange-mechanism to be responsible, with ice particles, cold
soil particles, etc. falling on supercooled turbulent water initiating the nucleation.
Splashing of water particles in cold air could freeze them and generate nucleus for
secondary nucleation. With ice particles in the
flow, secondary nucleation would cause
them to multiply and produce high concentration of frazil 2
fl
ice.
3.1.3 Ice Formation in Lakes
Ice crystal lattices are optically uniaxial. They start to grow on the lake surface into needle or
platelet shapes with the c-axis perpendicular to the axis of the needle or to the plane of the
platelet, as favoured by the anisotropic thermal conductivity of ice crystals. Overlain
together the platelets form macrocrystals, which are optically like single crystals but
structurally multiple crystals. In the geophysics of natural surface waters, the term ice
crystal or grain refers to single crystals or macrocrystals equivalently. These crystals are
classi
ed in terms of their size, shape and c-axis orientation (e.g., Michel 1978) (Table 3.1 ).
The crystal size ranges from less than a millimetre to 1 m, and the common dimensions
are 1 mm
10 cm. The size as well as the shape and orientation change in a continuous
manner but they are absorbed into a few ideal classes. In general, the slower the ice grows
the larger will the crystals be, and therefore the crystal size increases with depth. The main
shape classes are granular and columnar, and in the beginning of ice formation there is a
larger variability in shape with needle- and plate-like structures. The origin of the water
-
Table 3.1 Classification of ice
crystals on the basis of size,
shape and orientation
Crystal size
Fine
<1 mm
Medium
1 - 5mm
Large
5 - 20 mm
Extra large
>20 mm
Giant
meters
Crystal shape
Needles, plates
Granular (linear dimensions all of the same magnitude)
Columnar: elongated vertically
Crystal c-axis orientation
Vertical
Horizontal (random/aligned in 2D)
Random in 3D
2 Frazil comes the French word fraisil, which refers to coal cinders.
 
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