It is actually only in the cooler winter months when the water is warmer than the air, that most of the evaporation from the Great Lakes occurs. This can be exacerbated with low ice coverage. Alarmingly, between 1973 to 2010, spatial extent of ice coverage decreased in the Great Lakes by about 71% - impacting Georgian Bay water levels without question. But there is much more to it than that.The usual assumption around low ice cover would be that the Great Lakes would experience more evaporation since there is no 'lid' to keep the water in when the cold air drafts across the warmer than usual water. That's not really the only factor. Think of the lake as a large heat storage battery. This is why the Great Lakes can actually create their own microclimate. The hot summer sun warms the water throughout the warmer months. In the late fall and winter, when a cold arctic blast sweeps across the surface, the cold air sucks heat out of the lake in the form of evaporating water. This is where lake effect snow comes from. This is not always the case as we've seen from recent falls and winters. Understanding changes in Great Lakes evaporation is a new field of study since the scientific instrumentation has only recently been installed. Increased surface water temperature 1 and warmer than average air temperatures are resulting in minimal evaporation rates due to a lower thermodynamic energy difference during the fall and winter months.
2 general cases to consider
Case 1 - a warm/low ice winter
- 1. When the water is for example 5 degrees and the air is 5 degrees there is again no thermodynamic force to drive the water into the air. So the water doesn’t evaporate.
- 2. There isn’t much spring snow melt or lake ice melting, so the sun’s heat actually goes immediately into warming the water that continues to gain heat throughout the summer.
- 3. Later - we can imagine in this case, that the fall water temperature will be hotter. Then, if the winter brings cold dry air from the arctic, there will be more thermodynamic energy driving the warm water into the cold, dry air before any state change and so, more evaporation occurs.
Case 2 - a cold/high ice winter
- 1. The other important piece of this puzzle is “state change” or “phase transition”. Freezing liquid water to form solid ice requires energy to be removed from the water. This means that even without a change of temperature, energy moves from the water into the air. And once that is completed, the heat in the liquid water now covered by the ice is insulated from the air.
- 2. Next, the spring melt and lake ice melting absorbs energy even without the temperature rising above zero. Then, once the phase transition is completed, the water temperature starts to increase.
- 3. More phase change energy means less evaporation. This could lead to more ice in the subsequent season and higher water levels.