<P> Land below the glaciers "rebounded" as it was uncovered . Because the glaciers covered some areas longer than others, this glacial rebound occurred at different rates . </P> <P> The Great Lakes have a humid continental climate, Köppen climate classification Dfa (in southern areas) and Dfb (in northern parts) with varying influences from air masses from other regions including dry, cold Arctic systems, mild Pacific air masses from the West, and warm, wet tropical systems from the south and the Gulf of Mexico . The lakes themselves also have a moderating effect on the climate; they can also increase precipitation totals and produce lake effect snowfall . </P> <P> The most well - known winter effect of the Great Lakes on regional weather is the lake effect in snowfall, which is sometimes very localized . Even late in winter, the lakes often have no icepack in the middle . The prevailing winds from the west pick up the air and moisture from the lake surface, which is slightly warmer in relation to the cold surface winds above . As the slightly warmer, moist air passes over the colder land surface, the moisture often produces concentrated, heavy snowfall that sets up in bands or "streamers". This is similar to the effect of warmer air dropping snow as it passes over mountain ranges . During freezing weather with high winds, the "snow belts" receive regular snow fall from this localized weather pattern, especially along the eastern shores of the lakes . Snow belts are found in Wisconsin, Michigan, Ohio, Pennsylvania, and New York, United States; and Ontario, Canada . </P> <P> The lakes also moderate seasonal temperatures to some degree, but not with as large an influence as do large oceans; they absorb heat and cool the air in summer, then slowly radiate that heat in autumn . They protect against frost during transitional weather, and keep the summertime temperatures cooler than further inland . This effect can be very localized and overridden by offshore wind patterns . This temperature buffering produces areas known as "fruit belts", where fruit can be produced that is typically grown much farther south . For instance, Western Michigan has apple and cherry orchards, and vineyards cultivated adjacent to the lake shore as far north as the Grand Traverse Bay and Nottawasaga Bay in central Ontario . The eastern shore of Lake Michigan and the southern shore of Lake Erie have many successful wineries because of the moderating effect, as does the Niagara Peninsula between Lake Erie and Lake Ontario . A similar phenomenon allows wineries to flourish in the Finger Lakes region of New York, as well as in Prince Edward County, Ontario on Lake Ontario's northeast shore . Related to the lake effect is the regular occurrence of fog over medium - sized areas, particularly along the shorelines of the lakes . This is most noticeable along Lake Superior's shores . </P>

What is at the bottom of the great lakes