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Why do many deep lakes form warm and cool layers during summer?

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lake Lake Ann in North Cascades National Park, Washington, U.S., viewed from the park's Maple Loop Trail. The North Cascades National Park is a large wilderness area that preserves majestic mountain scenery, snowfields, glaciers, and other unique natural features.

During the summer, deep lakes form distinct warm and cool layers due to a process called vertical stratification. As solar radiation warms the surface of the lake, the density of the surface water decreases. This warmer, less dense water forms the epilimnion, which remains separated from the colder, denser water in the deep layer known as the hypolimnion.123

The two layers are divided by a zone of rapid temperature change called the thermocline. Because the warm surface water is significantly less dense than the cold water below, it resists mixing, effectively trapping the cooler water at the bottom. While wind action can mechanically mix the upper epilimnion, it is generally insufficient to overcome the density barrier and mix the entire lake during the summer months. This stable, two-layered system persists until the onset of autumn, when cooling surface temperatures increase the water's density, eventually leading to a complete turnover or circulation of the lake's water column.124