Ice cover on North America's Great Lakes reached a whopping 88 percent in mid-February… levels not observed since 1994. The average maximum ice extent since 1973 is just over 50 percent. It has surpassed 80 percent just five times in four decades. The lowest average ice extent occurred in 2002, when only 9.5 percent of the lakes froze.
It dropped a bit since then due to warmth… especially over the weekend… but as of yesterday it was back to near 80%... and looks like certainly more is ahead with the cold expected to linger in the north. This time of the year lake ice coverage changes very rapidly due to slight changes in temperatures or strong winds.
Creating the large and increased areal coverage of ice this winter has been persistently low temperatures across the Great Lakes region. Cold air and winds remove heat from the fresh water until it reaches the freezing point… at which point ice begins to form on the surface. Low temperatures are the dominant mechanism for thickening the ice… but secondary factors like clouds… snow and wind also play a role.
And the frozen lakes have implications well beyond the water surface and the winter. Extensive ice cover on the Great Lakes can change the patterns and amounts of the famous "lake effect" snowfall in the region. When the lakes are mostly ice free… cold west or north winds blow over the warmer water… pick up moisture and then drop snow on the lee side of the lakes (usually eastern and southern shores).
When the lakes freeze… the effect generally shuts down. However… the region is still receiving a fair amount of snow this winter due to general weather patterns… low pressure systems… etc.
Lake levels could be affected in the summer. Winter ice cover generally reduces evaporation during winter months. If that turns out to be the case in 2014… it would be good news for local water supplies as well as for shipping and recreational use.
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