How cold is the bottom of a lake in winter?

The less dense water freezes on the surface at 0°C (32°F), forming ice. The water actually becomes warmer nearer the lake bottom. In a deep lake, the bottom water temperature is 4°C (39°F), the densest water. Ice and snow cover the lake during the winter forming an insulating blanket.



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Three distinct layers develop: The top layer stays warm at around 65–75 degrees F (18.8–24.5 degrees C). The middle layer drops dramatically, usually to 45–65 degrees F (7.4–18.8 degrees C). The bottom layer is the coldest, staying at around 39–45 degrees F (4.0–7.4 degrees C).

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In summer, the top of the lake becomes warmer than the lower layers. You've probably noticed this when swimming in a lake in summer - your shoulders feel like they're in a warm bath while your feet are chilled. Since warm water is less dense that colder water, it stays on top of the lake surface.

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Lakes have layers Water temperatures also play a role in water density. Warm water is less dense meaning it is lighter and stays toward the top of the lake. The colder, heavier water is found at the bottom.

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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.

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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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When water cools down to 0 degrees Celsius (0°C), ice begins to form and floats on top of warmer water; water at the bottom of a lake or river is typically 4°C (if it is not frozen). During the summer, the opposite occurs as warmer water floats on top of colder water. Many factors can affect water temperature.

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But, in winter some lake surfaces can get very cold. When this happens, the surface water becomes more dense than the deeper water with a more constant year-round temperature (which is now warmer than the surface), and the lake turns, when the colder surface water sinks to the lake bottom.

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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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The Profundal Zone or Hypolimnion (cold water region) can be found below the Euphotic Zone. This zone is located under the thermocline where the sunlight does not penetrate the water. Again, the size of this zone depends on the age and water clarity of the pond or lake.

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It's all part of what makes weather, the unequal heating of the Earth's surface. That is why over the next couple of months you will hear us often say, “cooler by the lake.” Then again, as boaters and people who enjoy our beautiful waters know, it is always cooler by the lake.

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Keep it safe Particularly if you're a beginning cold water swimmer, have someone with you. The cold water can increase the likelihood of heart problems or cramps that even experienced pool swimmers aren't expecting. If you're not a confident swimmer, it's better to start in warmer and predictably shallow waters.

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Our bodies adjust best to the cold water if you slowly wade in. A quick entry into especially cold water can be a shock that leaves you gasping and could lead to drowning. A slow entry is key. Once you're up to your shoulders, dunk in all the way a few times and you're good to go.

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If you are getting 50-55 degrees Fahrenheit, turnover may be nearing or just starting. By 45-50 degrees Fahrenheit the lake is making the change rapidly, and fall turnover is complete at around 40.

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As the density of the water rises, so does its temperature. At its deepest point the water in Lake Lunz remains constant at four Celsius. A lake more than one meter deep will never freeze completely, meaning there's plenty of space for animal and plant life to survive the winter.

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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.

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