So, although one year may be warmer than the one before, some recent winters were icy cold. In 2013-14, the polar vortex carried frigid air from the Arctic into the continental U.S., and the cold stretched well south of the Great Lakes. Total ice cover on the Great Lakes spanned more than 90 percent, growing so thick in some places that the augers ice anglers used couldn't reach the water.

Maybe it gets cold enough for ice to form early in the winter -- but if a stretch of bitter wind keeps the water's surface churning, ice will form later. Maybe the summer before was extra warm, zapping enough extra heat into the water that it takes it extra long to cool off and get to the point when it can start to freeze. Maybe a bunch of snow falls early in the season, insulating the ice from the top and, counterintuitively, keeping it from growing quickly through the cold temperatures.