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R. A. Assel
F. H. Quinn


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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