Freezing and Lake Ice

by Lake Champlain Committee Staff Scientist Mike Winslow

Pancake ice. Photo by Mike Winslow

By late December shallow areas of Lake Champlain have frozen over. Ice fishermen will set up south of the Crown Point ferry/bridge crossing while just north open water remains. Along the Rt. 2 causeway Mallets Bay to the south will be frozen while open water persists to the north. This discrepancy between different parts of the lake points to some interesting observations about how large lakes cool and freeze.

 Most of the lakes’ heat energy is lost by direct convection to colder air masses sitting over the water. A light breeze facilitates heat loss from the surface by moving the just warmed air and replacing it with more cold air. On the other hand, a strong wind mixes water layers and brings up more heat stored in the deeper water.

 Once the water reaches a temperature at which it can freeze it does not freeze immediately; it must first expend excess energy. Liquid water has more energy than frozen water. That energy keeps the water molecules moving. In order to freeze the water’s energy must be released, and the release occurs without a change in temperature. The energy that must be released is known as the latent heat of fusion. During the entire time that water is freezing its temperature remains the same, 0o C. Once the water has frozen, then the temperature of the ice can again decrease below 0o C.

 Freezing occurs only where the full vertical profile of lake water is below 2o C. In deeper parts of the lake, water well below the surface can hold on to some of its summer heat and stay above 2o C. Thus, a bubble of relatively warm water sets up in the deeper waters. As winter progresses and the lake continues to cool, the line of 2oC water continues to progress toward the deeper parts of the lake.

 Eventually, the cold water layer sitting atop the bubble grows thicker and heavier. This presses down on the bubble and forces warmer water into the shallow regions of the lake as well. As a result, the rate of freezing can slow for a period of time and some of the ice that formed early in the season may even disappear. All of this is mediated by the duration and extremity of cold air sitting over the lake during the time of freezing.

 Lake water goes through many stages before finally becoming a thick sheet upon which we can skate or walk. Tiny needle-shaped crystals known as frazil form and orient randomly at the outset of freezing. As the frazil becomes more abundant, the crystals begin to coagulate. Under calm conditions they can form continuous sheets of thin transparent flexible ice known as nilas. This is the type of ice you might see as a skim on puddles after the first cold frosts of autumn. Nilas may grow to about 10 centimeters thick. After that, instead of growing vertically, water molecules begin to freeze on the bottom of the existing ice sheet increasing its thickness.

 Alternatively, with even slight turbulence grease ice can form when frazil crystals clump together, forming a thin soupy layer resembling an oil slick. Over time, wind and waves can force grease ice and slush (a floating mass formed from snow and liquid water) to accumulate into spongy pieces several inches in size known as shuga. Further compression of shuga leads to larger plates called pancake ice. Pancake ice can be distinguished from shuga because the pieces have elevated rims of uniform height caused by the piling of frozen crystals at the edges when pieces bang into one another. Shuga is not dense enough to form this feature. Pancake ice also forms when ice sheets break apart.

 Under certain conditions ice can form on the bottom of the lake. This might happen if water becomes supercooled, meaning the water temperature drops below its freezing point but the water remains liquid. Supercooling occurs when there are no particles in the water to promote the formation of ice crystals or when water is cooled very quickly. In Lake Champlain supercooled waters are most likely to be found when air temperatures are below minus 6o C and there is a great deal of turbulence to prevent crystal formation Turbulence caused by strong winds can mix the supercooled water throughout the water depth. Frazil ice crystals form in the super cooled water and, because the crystals lack buoyancy, they are carried to the bottom. There it can adhere to objects in the water. The ice crystals accumulate on one another and grow upon the bottom of the lake forming anchor ice.

 At the edge of ice waves and wind continually reorder the ice sheet. Pancake pieces break off and then refreeze. Large slabs of ice are thrust up over the sheet below. The ice contracts as it cools and expands when it warms. Throughout the winter continual observation of lake ice will reveal numerous shapes, colors and phenomena worth a little study and exploration.

Lake Look is a monthly natural history column produced by the Lake Champlain Committee (LCC). Formed in 1963, LCC is the only bi-state organization solely dedicated to protecting Lake Champlain’s health and accessibility. LCC uses science-based advocacy, education, and collaborative action to protect and restore water quality, safeguard natural habitats, foster stewardship, and ensure recreational access.

Get involved by joining LCC using our website secure form (at www.lakechamplaincommittee.org), or mail your contribution (Lake Champlain Committee, 208 Flynn Avenue - BLDG 3 - STUDIO 3-F, Burlington, VT 05401), or contact us at (802) 658-1414, or lcc@ lakechamplaincommittee.org for more information.