The bar graph above illustrates the tendency of several lakes covered in ice during two analyzed timeframes. An average lake ice cover displayed both peak and trough throughout the periods: from 1973 to 1992 as well as from 1992 to 2012 in North America.
Overall, the highest level of ice cover in the first period, until 1992, was fixed in the Erie area, reaching 30%. During that time, Huron and Superior showed a similar situation as Erie and both exceeded a level of 25%. Ontario was the least ice-covered lake in North America, slightly crossed 5% before 1992.
In comparison with the second timeframe, the same high position was followed by Erie, however from 1992 to 2012, its percentages fell by 2-3 points. It could not be forgotten to mention, that after 1992 there was a sharp decrease in ice cover on Superior Lake, moving approximately from 26% to 14%. Moreover, Michigan took place as a medium in that statistic data with a noticeable fall after 1992.