Structure and Properties of Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 59

molecules in ice crystals is the lake water body itself or precipitation. The latter case is
also called meteoric ice.
The first ice layer on lake surface is called primary ice. Ice formation may initiate in

three different modes of heterogeneous nucleation: nucleation on suspended particles at
the surface in calm or laminar

flow conditions, nucleation on suspended particles under
the surface in turbulent conditions as frazil ice, or nucleation on snow or ice nuclei
precipitating onto the water surface. In frazil ice formation, supercooling is limited to less
than 0.1

fl

C. The physics of the primary nucleation is partly unclear (see above), but once
the crystallization has started, secondary nucleation leads to a fast growth of the frazil ice
volume. Secondary ice forms beneath the primary ice layer. In a lake ice sheet this is
congelation ice, where crystallization takes place at the bottom of the ice sheet. Therefore
no supercooling is needed any more. Superimposed ice forms on top of the primary ice.
This includes snow-ice formation, freezing of surface ponds, and surface hoar formation.
The source of the superimposed ice is slush, 3 liquid water ponds, or atmospheric moisture.
Liquid water ponds originate from meltwater or liquid precipitation and they are thin, as
surface hoar layers as well, both of the order of 1 cm. The main contribution to the
superimposed ice layer is due to freezing of slush. Snow-ice crystals are small as in frazil
ice but occasionally larger snow crystals or frozen water pockets are found.
In brackish or saline lakes, ice crystals are in size and shape much as freshwater ice
crystals. No ice structure analyses are known to the present author for hypersaline lakes.
Most of the investigations of saline ice are from the cold ocean

°

showing that
there are two important differences caused by the large amount of dissolved substances in
the parent water. In sea-ice the crystal boundaries are jagged, and between the single
crystal platelets inside macrocrystals there are brine inclusions (Weeks 1998). Brackish
ice has been examined from

— sea-ice —

field data in the Baltic Sea (Palosuo 1961; Weeks et al. 1990;
Kawamura et al. 2001), and the results show that the transition between freshwater ice
type and sea ice type takes place when the salinity of the parent water is 1

.Itis
anticipated that in brackish and saline lakes the situation is much as in seawater. Weeks
and Lofgren (1967) have also shown that ice formed of sodium chloride solution (with salt
concentration as in sea water) is similar to sea ice, when the salt concentrations are similar.

2 ‰

-

3.1.4 Physical Properties of Lake Ice

The properties of freshwater lake ice depend primarily on its crystal structure and gas
content. The basic properties of this ice with the sensitivity to temperature are given in
Table 3.2 . More details are shown in Annex 1. For most properties the sensitivity to
temperature is not very high (less than 5 % per 10

°

fixed values
can be used. The bottom temperature of ice is at the freezing point while the surface
temperature depends on the air temperature and the thicknesses of snow and ice. In the case

C), and in many applications

3 Slush is water-saturated snow.