Average ice-out dates for Minnesota lakes. The number of lakes varies from 7 in the early part of the record to over 60 lakes in recent times. All values were normalized by applying an iterative process to form an average bias for each lake from a theoretical average lake for its location. Ice-out dates were latest in the mid-20th century through the 1970s. Since then, there has been a trend toward earlier ice-out dates. Figure courtesy of Minnesota Department of Natural Resources.