Are lakes colder at the bottom?

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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The gravitational weight of all the water higher up in the lake presses down on the water deep in the lake. The pressure allows the water near the bottom of the lake to get cold without expanding and rising. Because of the pressure, the water at the bottom of deep lakes can become cold without freezing to ice.

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When the upper water layers warm in the summer months, they become separated from deep water by a transition zone known as a thermocline. In a thermocline, the temperature decreases rapidly with small increases in depth. This phenomenon linking temperature change with depth is called temperature stratification.

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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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The hypolimnion is the bottom layer and is colder and denser than either the epilimnion or metalimnion. When a lake or reservoir is thermally stratified, the hypolimnion becomes largely isolated from atmospheric conditions and is often referred to as being stagnant.

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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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We now have a micro (small) scale circulation — the lake breeze. This lake breeze that develops in the spring can cause a 30 degree difference in temperature over a few miles. It can oscillate back and forth, where you a warm one minute, but cold the next. We see this on a grand scale as well around the globe.

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Turnover is like being at the mall … or at a fair. Turnover is a natural way the lake cleans up harmful bacteria and algae. It carries dead algae down into the depths of the lake where there is less sunlight, helping to prevent algae growth.

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The seasons at a glance Top tip: In the bleak mid-winter, when it's really cold and settled, go for the bottom half of the middle third, occasionally rising up in the water if the day is bright. The lake warms up from the surface downwards, slowly heating all the way through the water column.

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The coldest lake in the world is Lake Baikal in Russia. It also happens to be the largest freshwater lake in the world. Lake Baikal is in Southern Siberia. The lake freezes during winter, becoming an ice lake.

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