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Ice Cover Lacking on Lake

Ice Volcano

An ice volcano along Lake Superior’s shore erupts as waves crash. Photo: Dan Urbanski

Winter in the northland:

  • the sun sets by 4:30 p.m.
  • parkas and balaclavas obscure the human form
  • inland lakes are completely frozen
  • Lake Superior continues to lap the shore.

Anyone who knows Lake Superior knows that this inland sea behaves as if air temperatures measured in negative numbers are of no consequence. A large capacity to retain heat coupled with a vast and animated surface inhibits ice from forming well after other lakes support SUVs and ice fishing villages. This winter Lake Superior and other Great Lakes barely froze at all. This has consequences for the lakes and the humans and animals that depend upon them.

Ice puts a lid on evaporation, which crescendos in Lake Superior from August to a December climax. Unlike typical lakes, Superior evaporates fastest in fall and early winter, as cold, dry nor’westers whistle over the chop, still relatively warm with the lake’s radiant heat, and suck up water like a sponge. (For each 18° F (8° C) increase, air can hold two times more water.) Evaporation wicks over a foot (300 mm) of water off Lake Superior between November and February, throwing much of it on the South Shore as lake-effect snow.

The formation, duration, and extent of ice cover influences the water balance of Lake Superior. An ice sheet keeps water in the lake. As the lake becomes encrusted, less water is gulped up by the thirsty air. In March, when ice cover generally reaches an apex, evaporation ranges from less than an inch (20 mm) to over 3 inches (87 mm), reflecting temperatures, humidity, and ice cover. While loss of an inch in lake depth might not seem like much, it can have a big impact on the shipping industry. For large vessels, each inch equals about 270 tons of cargo.

Although ice keeps more water in the lake for cargo transportation, it bars shipping for about two months each winter. Superior’s ice also complicates hydropower production, damages shoreline structures, and allows daring anglers to pursue their passion.

Ice fishermen, whitefish eggs, and diatoms fare better when Superior freezes. Although ice limits photosynthesis by diminishing the south-slanting rays of winter’s short days, in many years it provides unique fishing opportunities. Once the ice gets thick enough, portable fish houses intermittently appear in nearshore areas from Duluth to Two Harbors.

“Lake Superior’s ice fishery is highly variable,” said Joe Ostazeski of the Minnesota Department of Natural Resources Lake Superior Area Fisheries Office. “It depends on the ice conditions. If the ice is thick enough that people can go a quarter mile or more out, they start getting lake trout. More commonly, they stay within 20 yards and fish for Kamloops and Coho, which stay tight to the shore in winter.”

Beyond its functional properties, Superior’s ice can take your breath away (assuming the bitter winds haven’t first). The formations can take a striking variety of configurations, from acres of glassy black ice to rafted sculptures over 10 feet high. One of the first ice types to form on Lake Superior is an ice foot, a conglomerate of ice balls and wave spray that freezes to the shore. Ice volcanoes are one of the most fantastic formations.

These “volcanoes” are born as high surf slams against the face of an ice shelf, sending a mighty pulse of water underneath that bursts from cracks or weak points. Ice chunks and lake-spray build around the rim of these sporadically erupting cracks to form conical mountains sometimes exceeding 26 feet (8 meters) measured from base to peak. These late-winter phenomenon require ice on the lake, high surf, and below-freezing air temperatures. Whether cones form depends on wind direction and the geography of the lake floor. Although ice volcanoes can form on rock reefs and sand bars, protected bays or shorelines rarely experience enough wave energy to generate these startling formations.

Lake Superior is never in a rush to freeze and rarely freezes all the way (the last time was in 1994; almost in 2003). Understanding that ice cover is sensitive to climate trends, scientists have been monitoring the duration and extent of ice on Lake Superior since 1973. If you have Internet access, ice data through 2002 is presented in charts and time-series animation on the National Oceanic and Atmospheric Administration’s Great Lakes Ice Atlas. Current Lake Superior conditions are also available on the Great Lakes web site.

By Sharon Moen
April 2006

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