Structure and Properties of Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 55

Ice is optically uniaxial with the c-axis as the optical axis. Light goes through unaf-
fected in the direction of the optical axis, while in other directions birefringence (double
refraction) takes place. Thermal conductivity is highest along the basal plane. Shear
deformation is easiest parallel to the basal plane since then the number of bonds to be
broken for shear is at minimum. Therefore this plane is also called slip plane or shear
plane in ice mechanics.

3.1.2 Ice Nucleation

When liquid water is cooled, the nucleation temperature is always below the freezing
point (Fig. 3.2 ). Certain amount of supercooling is needed to start nucleation, and
thereafter the

is recovered and the temperature returns to the freezing point.
Then, in the solid state, temperature decreases if heat removal continues. Homogeneous
and heterogeneous nucleation modes exist in the primary nucleation (see Michel 1978). In
the former case, very strong supercooling (down to

' extra cold '

C) would be required in pure
fresh water, but this is not observed in natural lakes. Instead, heterogeneous nucleation
takes place with suspended particles acting as the crystallization seeds, and supercooling
remains small. Also snow or ice crystals falling on a supercooled lake surface may initiate
nucleation. In the secondary nucleation, crystals grow on existing ice surfaces. The latent
heat of freezing is large (333.5 kJ kg − 1 ) and severely limits the volume of ice produced in
nature. For a heat loss of Q, an ice layer of thickness h = Qt/(

− 40 °

ρ i L f ) is produced in time t;
e.g., for Q = 100 W m − 2 and t = 1 d, we have h = 28 mm (Fig. 3.3 ).
To build a crystal in a homogeneous is free energy, is

required to form the surface and the bulk particle. This energy and the minimum radius of
stable crystals are (Michel 1978), respectively,

fl fluid, work ʔ G, called the Gibb '

Fig. 3.2 Schematic cooling
curve of a liquid with heat
being removed at a constant
rate; T is the temperature, T * is

the temperature of phase
equilibrium, and ʔ T 0 is the
degree of supercooling.
Modified from Michel (1978)