Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 165

FIGURE 4.13 Mt. Washington summit observatory covered in rime. (Photo courtesy Mount Washing-
ton Observatory.)
FREEZING OF LAKES AND PONDS
Another mystery of nature that people often take for granted is the fact that ice floats
in its own liquid! Of commonly found compounds, only ammonia shares this trait with
water. Lakes, ponds, and other relatively quiet bodies of water in high mountain en-
vironments often freeze over in the winter, but only very small lakes and ponds freeze
completely solid. As the lake water near the surface cools with the approach of winter,
its density increases and the cooled water sinks. When the temperature of the lake wa-
ter cools below +4°C (39°F), the colder water becomes less dense, as the molecules
begin to take on the expanded volume associated with the crystal lattice of ice. During
this process, the lake water will completely overturn (i.e., exchange bottom water with
top water). The near-freezing water, with its abundance of freezing nuclei, now floats to
the surface and the crystal lattices merge to form of a thin sheet of skim ice. With in-
creased thickening, the so-called “C” crystal axis of the ice becomes oriented vertically,
producing a dark ice called black ice (or candle ice because of the pronounced vertic-
al ice columns observed during melt-out). The ice thickens rapidly at first, but the rate
slows down because of a self-insulating feedback loop. A second layer in the lake ice
pack forms when snow falls on the black ice and depresses it isostatically into the wa-
ter. Cracks form in the black ice that allow lake water to flood the overlying snow, pro-
ducing a frothy layer of white ice (Gray and Male 1981) (Fig. 4.14). While this process
may seem merely an interesting curiosity, it has far-reaching consequences for aquatic