Early winter freezes and thaws, occurring now in the Upper Peninsula and in the northern Lower Peninsula, render ice unsafe for any type of traffic, especially in December and early-January. Temperature fluctuations typically occurring in March and April will again make for uncertain ice. Be suspicious of gray, dark or porous spots in the ice as these may be soft areas. Ice is generally strongest where it is hard and blue. Heavy snow cover insulates ice and prevents it from freezing as evenly and as quickly as it would if the snow weren't there.
Snow cover is also deceptive and makes evaluating the ice cover difficult.
Be especially wary of river ice, as it can be highly variable in thickness due to the erosive action of the underlying river current. Carry a couple of large nails and a length of light nylon rope. Should you break through the ice, try not to panic. Use the nails or your hands to gain a hold on the unbroken surface of the ice, and advance by kicking your feet. Roll away from the point where you broke through until you are on solid ice.