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R. A. Assel and F. H. Quinn

Abstract

The. formation of ice cover on the Great Lakes during the 1976–77 winter was unusual because of the early onset and continuation of below normal air temperatures. The severe winter produced a particularly extensive ice cover in the southern half of Lake Michigan. During the height of the winter, in early February 1977, the lake was almost entirely frozen over. To put the winter in its proper perspective, temperature records starting in 1897 and ice-cover records beginning in the 1962–63 winter were analyzed to classify winter severity and to examine the relationship between winter severity and maximum ice extent an Lake Michigan. The winters were classified by freezing degree-days into five categories.. severe, severe than normal, milder than normal and mild. The classification indicates that the winter of 1976–77 was one of the four coldest in the past 80 years. The analysis also shows that the past 15–20 winters have been colder than the normal established by the 80-year data base. As well-documented ice-cover records of Lake Michigan have only been collected during the past 15 years, existing ice-cover normals based on these records are probably biased toward the severe condition. The analysis also shows that extensive ice cover (in excess of 50% of the total lake's surface area) develops on Lake Michigan only when the southern subregion of the lake experiences a severe winter.

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R. A. Assel, J. E. Janowiak, D. Boyce, C. O'Connors, F. H. Quinn, and D. C. Norton

Winter 1997/98 occurred during one of the strongest warm El Niño events, and the Great Lakes experienced one of the least extensive ice covers of this century. Seasonal maximum ice cover for the combined area of the Great Lakes was the lowest on record (15%) relative to winters since 1963, a distinction formerly held by winter 1982/83 (25%), which was also an exceptionally strong El Niño winter. Maximum ice covers set new lows in winter 1997/98 for Lakes Erie (5%), Ontario (6%), and Superior (11%), tied the all-time low for Lake Huron (29%), and came close to tying the all-time low on Lake Michigan (15%; all-time low is 13%). Here the authors compare seasonal progression of lake-averaged ice cover for winter 1982/83, winter 1997/98, and a 20-winter normal (1960–79) derived from the NOAA Great Lakes Ice Atlas and discuss the 1997/98 ice cover in detail. Winter air temperatures in the Great Lakes were at or near record high levels, storms were displaced farther to the south over eastern North America, and precipitation was below average in the northern portion of the Great Lakes region. The Northern Hemispheric synoptic flow patterns responsible for this winter weather, the Great Lakes winter severity over the past two centuries, and impacts of this mild winter are briefly discussed.

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