The Great Lakes often act like large insulators—slow to warm up, slow to cool down—and can have a dramatic effect on snowfall. Dry winter air travels over the Great Lakes on the prevailing westerly winds, absorbing moisture. When this air hits land, it dumps its precipitation in the form of snow. Meteorologists and locals alike refer to this phenomenon as lake-effect snow. This phenomenon most often affects the Superior Highlands, which explains why many of the U.P.’s successful ski resorts are clustered near Iron Mountain along the western edge of the peninsula.
The prevailing western breezes also affect Great Lakes water temperatures, most noticeably on Lake Michigan. In summer, the warm surface waters tend to blow into the eastern Lake Michigan beaches along the Lower Peninsula. While Lake Michigan is rarely warm enough for much swimming on the Wisconsin side, it can be exceedingly pleasant in the Lower Peninsula and at least tolerable in the U.P., as evidenced by the hundreds of popular swimming beaches lining the west side of the state.
Lake Superior is altogether different. Water temperatures in this northern lake rarely climb out of the 40s, save for the occasional shallow bay. “No one’s ever drowned in Lake Superior,” the saying goes. “They all die of hypothermia first.”
Freshwater freezes more quickly than saltwater, and the Great Lakes often ice over several miles out from shore. The commercial shipping season shuts down from January 15 to March 25, but persistent ice can hamper it for much longer. During the winter of 2013-2014 ice clogged the Straits of Mackinac well into April, making it difficult for Mackinac Island to prepare for the subsequent tourism season. Ice floes on Lake Superior persisted well into May. Commercial ships can regularly cut through ice up to a foot thick. Thicker ice requires the services of the Coast Guard’s Mackinaw, a 240-foot heavily reinforced ship specifically designed for ramming a passage through ice. The Mackinaw can “back and ram” its way through walls of ice as high as 10 feet.