How do you tell if a lake is frozen enough to walk on?
As a general rule of thumb, ice is safe to walk on when there is 4 inches or more of clear ice. It is important to understand that different types of ice varying in their strength. Clear blue or black ice is the strongest.
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On average, it takes four days of below freezing temperatures to form ice that is safe. Also, this guideline is for ponds and lakes. Frozen streams should never be walked on as they freeze unevenly due to the movement of the water. Another thing to know is that clear ice is the strongest.
Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below. Our winters aren't long or cold enough to completely freeze most local water bodies. This process of lakes turning over is crtically important to the life in the lake.
Thick ice weakens slowly, but it also recovers slowly. While two inches of ice might last only two hours in moderate thaw conditions, a foot or more of ice can last several days.
Ice may form quickly when temperatures tumble, but it takes more time than you might think for ice to reach the four-inch thickness that experts recommend. On average, it takes four days of below freezing temperatures to form ice that is safe. Also, this guideline is for ponds and lakes.
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.
Melting of lake ice usually occurs first near the shorelines or near the mouths of streams. At these points of contact with inflowing warm water, the ice melts faster than it does at central lake locations, where most melting is caused by the transfer of heat from the atmosphere.
The sudden shock of the cold water can quickly impact your breathing, your heart rate and your blood pressure enough so that it can be life-threatening. You are then at a greater risk of drowning.
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.
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.