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Natural History Note – Lake Ice

Clear, cold, windless nights present ideal conditions for the actual freezing of ponds and lakes. The clear skies mean no cloud cover blanket to trap heat radiated away from the earth overnight. Colder air allows more heat to be transferred from the water. Absence of wind means that once the ice begins to form, wave action stirred by the wind will not interrupt the process.  

Different bodies of water freeze at different times, and three of the principle factors influencing which lakes freeze and which don’t are the lake's location, volume, and area. Obviously a lake in Florida is less likely to freeze than a lake in the Northeast, but the other aspect of location is altitude. Colder temperatures at higher elevations make lakes in the mountains more likely to freeze. Volume is another relatively obvious factor. It takes more time and colder air temperatures to draw the heat out of a large volume of water than it does a small volume of water. For that reason, fairly small lakes that are very deep will take much longer to freeze. The influence of lake area is a little less obvious, but the expanse of a flat treeless surface provides no hindrance to gathering winds, which can break up the ice as it forms. Thus, large lakes will freeze more slowly than small lakes of a similar volume.   

Freezing in very large lakes is usually preceded by a few good snow storms. When snow lands in the lake it warms and changes from a solid to a liquid form of water. Such a transformation requires more energy than a commensurate change in temperature without a change in phase. Thus the snow saps more of the lake’s energy reserves and brings a freeze-over closer.  

Two different types of ice cover can form depending on conditions at the time of freezing. Ice formation can be rapid with as much as five miles of ice forming on one river in only fifteen minutes. When a skin of ice forms all at once like this and then continues to grow from the initial start, it is called sheet ice. Sheet ice tends to be smooth and relatively homogenous, though cracks will appear. Alternatively, separate masses of ice can fuse together forming agglomeritic ice. Two conditions that promote formation of agglomeritic ice are when wave action or other turbulence prevents a continuous sheet from forming but it is so cold that freezing occurs anyway, or when sheet ice breaks up and then reforms.  

The sheet of ice over a lake is not static; it continues to grow, shrink, and move throughout the winter. Though ice is less dense than the water beneath, it is still subject to normal expansion and contraction with further changes in temperature. Three phenomena that demonstrate the dynamic nature of ice are pressure ridges, tension cracks, and ice ramparts. Each of these three phenomena form because of the expansion of ice as it warms.  

Pressure ridges result from the buckling of ice as it expands. Eventually the buckling leads to cracks, and one sheet of ice shoots up over another. The cracks form at right angles to the movement of the expanding ice. The process is similar to how mountains form.  

While pressure ridges form when two ice sheets push against one another, tension cracks form when the ice molecules pull apart. As the ice expands, the molecules within the sheet pull away from each other creating tension. The addition of even a slight amount of pressure, like from a walking person, can relieve the tension and allow a thin fissure to appear usually accompanied by a loud snap.  

Ice ramparts form when the expanding sheet of ice pushes against the shoreline rather than against more ice. The pressure from the expanding ice can force gravel and stones on to the land. Ice ramparts can cause severe damage to shoreline property or docks that may have been left in the water.   

Though ice ramparts provide a mechanism for moving sand and gravel toward shore during the winter, anchor ice can move them in the opposite direction. Anchor ice is that which attaches to the bottom of a lake or river.  As it forms, it adheres to sediment and pebbles. When the ice breaks free of the bottom it can lift and transport the sediment wherever the raft of ice floats. One study on Lake Michigan suggests anchor ice is responsible for removing 9.1 cubic feet of sand from each foot of beach!  

We flippantly toss around the phrase “it’s freezing” throughout the winter, but on average about every other year, at least in the last few decades, the main portion of Lake Champlain hasn’t frozen entirely (the Lake’s bays do freeze regularly). In the twenty years between 1988 and 2007 (the last year for which NOAA shows data), the lake froze nine times. In contrast, in the twenty year period between 1888 and 1907 the lake froze every year. So if you get a chance, take advantage of a day on Lake Champlain’s ice. Such days are getting harder to come by.