Re: Is lake ice thicker in the middle or close to shore? Ice on the edge is limited to the depth of the water at the edge. So it is always thicker towards the middle.
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If you can see the ground through the ice, then there's going to be warmer water under it. Out in the middle, the ice grows thicker over the cold dark depths. Also, ice always melts away from the shore first.
Ice formed over flowing water and currents is often dangerous. This is especially true near streams, bridges, and culverts. Also, the ice on outside river bends, and inlets and outlets of lakes and ponds is usually weaker due to the undermining effects of the faster current.
Ice is not considered safe to walk on until it is at least four inches thick. At that thickness, it is suitable for ice fishing, cross country skiing, and walking, and can support about 200 pounds. At five inches, it should be enough to hold a single snowmobile or ATV or about 800 pounds.
At some point, the ice layer itself on a lake will act as an insulator, preventing the cold air above the ice to remove heat from the unfrozen water below. This is why lakes don't typically freeze completely from top to bottom.
Avoid ice over the fastest current of moving water. Spots where streams enter and leave lakes are notorious for thin ice. Springs bringing water into a lake may create a thin spot that the locals should know about. Wind-scoured ice tends to be weaker than sheltered ice.
Lake ice freezes first at the surface starting at the edges or shoreline for two reasons. Water near the shore is typically shallower and contains less heat than deeper water so it can reach the freezing point faster than deeper water.
Since water is good at holding heat, the more water there is, the more heat it will hold. This is why large deep lakes take longer freeze and melt than small shallow lakes. Water freezes from the perimeter of the lake to the center.
Great Lakes that have completely frozen include Lake Superior, Lake Huron, Lake Erie, and Lake Ontario. Lake Michigan is the only Great Lake to have never frozen entirely.
In water that is around the freezing point, a person is likely to survive only 15 to 45 minutes with flotation and possibly up to an hour or so with flotation and protective gear before the brain and heart stop (Table 1). The surface temperature of Lake Superior in early to mid-summer is about 40 to 50 F.