The chart mention about the average percentage of five North American lakes covered in ice during two 20-year period between 1973-92 and 1913-2012 . Overall, the percentage ofice cover in Erie is the highest both two periods, it up to thirty percentages in first period and about twenty five a half in second period that differ the lowest in Ontario, there are less than five percentages in second period and more than five percentage in first period. There are approximately twenty five percentages in 1973-1992 in Superior Lake but the chart show that in 1993-2012, the average ice cover in Superior Lake were decresed nearly a half to about fifteen percentages that equal to Michigan in first period and decrese to around ten percentages in next period. Similarly, Hurun have changing in ice cover in the way as Michigan. The average percentage of ice cover in all lakes in seconds period are less than in the first period.