Great Lakes Article:

Less ice on Lake Superior
By Dan Schneider
Mining Gazette
Published February 16, 2008

HOUGHTON — Charts from the National Oceanic and Atmospheric Administration show less ice cover on Lake Superior than was present at this time last year, despite colder average surface water temperatures.

George Leshkevich, a research physical scientist at the NOAA’s Great Lakes Environmental Research Laboratory in Ann Arbor, said the latest ice charts released Thursday indicate only a small proportion of Lake Superior’s surface covered with ice.

“It does look like there’s still just shore ice, mainly now along the southern shore of the lake to Duluth around the Apostles and the Keweenaw and over to Whitefish Bay,” Leshkevich said, adding most bays along the southern shore are iced-in. “It looks like in comparison to last year, the chart from last year about the same day, same time, had more ice than this year.”

Last winter, lack of ice cover on Lake Superior enhanced the lake’s evaporation rate. Surface evaporation, along with long-term drought conditions, contributed to last summer’s near-record low water levels.

However, weather and hydrology experts say other factors besides ice cover play a role in the lake’s evaporation rate.

“I would say that last year at this time, there was a little more ice than there is now but that doesn’t necessarily mean there is more evaporation going on now,” said Keith Kompoltowicz, a meteorologist with the U.S. Army Corps of Engineers Detroit District. “The biggest thing with evaporation is there has to be a big difference between the air temperature and the lake temperature.”

When cold air sweeps down from the north, it pulls water out of the relatively warmer Lake Superior. The greater the temperature difference, the greater the rate of evaporation.

Currently, the average surface temperature across Lake Superior is about 33.9 degrees Fahrenheit, which is 0.6 degrees colder than last year, making the lake that much less susceptible to the northern air.

“Half a degree, it certainly can make a difference, but it’s kind of odd in a way that there was more ice cover” last winter, Leshkevich said.

He said wind and the waves it generates discourages the development of ice.

“Usually ice forms naturally when it’s kind of calm and water gets down to the freezing point,” Leshkevich said.

Kompoltowicz said ice cover has grown noticeably over the past few days, though it remains close to shore and in bays.

The last year Lake Superior saw “fairly extensive ice cover” was in 2003, Leshkevich said. He said 1996 and 1997 were the last years the lake had extensive and solid ice cover.

NOAA Hydrologist Cynthia Sellinger said a lot of the water that evaporates from Lake Superior falls onto land in the form of snow over the course of the winter. When that snow stays frozen until spring, the runoff seeps into aquifers and replenishes the big lake.

However, when periods of warm temperatures cause snow-melt while the ground remains frozen, puddles form. Sellinger said water evaporating out of these puddles often does not return to the Great Lakes.

She said this phenomenon is occurring with increasing regularity downstate, contributing to lower water levels in lakes Michigan and Huron.

On Thursday, the most recent day for which U.S. Army Corps of Engineers water level data is available, Lake Superior’s water level was at 600.5 feet above sea level. That’s about 9 inches higher than last year’s February average, though it remains 9 inches lower than the long-term average lake level for February. It is also about 7 inches lower than the chart datum level, the water level that is the basis for Lake Superior navigational charts, which is 601.1 feet above sea level.



Dan Schneider can be reached at dschneider@mininggazette.com