How cold does it have to be for a whole lake to freeze?

Before a lake can freeze over, its entire water column from top to bottom, must reach that magic temperature (39.2° F or 4° C). This natural cooling process is called fall overturn. It is a gradual process as the surface water slowly cools down and a larger and large layer of water can be mixed by the wind.



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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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Ice thickness can vary over a single body of water; it may be a foot thick in one location and only an inch or two a few feet away.

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Health risks Winter swimming can be dangerous to people who are not used to swimming in very cold water. After immersion in cold water the cold shock response will occur, causing an uncontrollable gasp for air. This is followed by hyperventilation, a longer period of more rapid breathing.

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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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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.

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Right when the water freezes to ice, the ice becomes significantly less dense than the water and continues to float on the lake's surface. Below 4° Celsius, water becomes less dense as it gets colder, causing water about to freeze to float to the top.

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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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Enough oxygen However, colder water can hold more dissolved gas than warmer water can, so water below freezing holds the most oxygen. Then, because fish metabolism has slowed down, they are using less oxygen. Some fish even go into a state called torpor that decreases their need for oxygen even more.

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At zero degrees, water and ice are in a state of thermodynamic equilibrium. Water molecules are released from the ice, while at the same time, ice molecules are formed from the water. Celler now adds a bit of table salt to the bowl and stirs. The temperature drops, but look at this, the water doesn't freeze.

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The high concentration of salt in ocean water lowers its freezing point from 32° F (0° C) to 28° F (-2° C). As a result, the ambient temperature must reach a lower point in order to freeze the ocean than to freeze freshwater lakes.

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The reason water freezes from the top down is because, unlike almost everything else, water gets less dense when it freezes. This is why ice cubes float in a drink.

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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.

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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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As a result, close to freezing, colder water floats to the top and the warmer water sinks to the bottom. The density of water as a function of temperature can be seen in the plot on the right. Eventually, the coldest water, which has floated to the top of the lake in wintry conditions, freezes to form a layer of ice.

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While ice fishing is the most common activity we see on frozen waters in Minnesota, that is not the only way to explore our frozen lakes on foot. Many Minnesotans enjoy snowshoeing, cross-country skiing, ice skating, iceboating, windsurfing, and kiteboarding among other activities on the ice.

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