How long does a lake take to thaw?

Because of the time required to melt ice that has thickened over the winter, the clearing of lake ice occurs some time after average daily air temperatures rise above freezing. Typically the lag is on the order of one month at latitude 50° N and about six weeks at 70° N.



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Moderate wind and warm weather: A day of moderate temperatures with some wind (15-25 mph) can do a lot of melting. Surface melting of a couple inches a day or more may occur. Puddle holes might form. Thin ice melts completely in pressure ridges and over gas holes.

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Warm wind is the primary driver behind top surface melting. For example, over a 24-hour period at 50°F, strong (20-30 mile per hour) winds can cause over two inches of ice loss. In the same period, heavy rain and only moderate wind result in just one inch of ice loss.

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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. This ice is formed from the lake water freezing.

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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.

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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.

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Do fish die in frozen lakes or in lakes that are partially frozen? Since fish are cold-blooded animals, they can survive because they are able to regulate their body temperature to match their environment. However, they could die if a body of water freezes over completely and remains frozen for an extended period.

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If water were most dense as a solid, lakes would freeze from the bottom up, eventually freezing solid. In that case, little or nothing would survive in the lake. Most lakes and ponds don't completely freeze because the ice (and eventually snow) on the surface acts to insulate the water below.

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Water under the ice typically stays very cold, but above freezing. Most lakes that form ice at the surface stay at about 4oC most of the winter months.

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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.

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Additionally, if ice forms too thickly on the surface of the water, it can prevent oxygen from diffusing into the water and causing widespread fish deaths. Another problem that can occur when a lake freezes is that of entrapment. If there are cracks or holes in the ice, fish may swim into them and become trapped.

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As a lake freezes, the ice floats on the surface, insulating the water below and keeping most lakes—and the fish within them—from freezing solid. You would still, however, be very cold. Lake fish have to lower their metabolism and enter a state called torpor to reduce their energy demands enough to survive the winter.

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On the surface of lakes, or other bodies of still freshwater, congelation ice is often called black Ice. This ice has frozen without many air bubbles trapped inside, making it transparent. Its transparency reveals the colour, usually black, of the water beneath it, hence the name.

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