The process of thermal stratification is most prevalent in the summer, and has a big influence on fish behaviour in waters that are affected.
In calm conditions, when wind-driven mixing is minimal, the surface layers of the water column are heated by the sun, and effectively become separated from the cooler water away from the surface.
The boundary between the two layers, known as the thermocline. The upper layer of water is warm and rich in oxygen, while the lower layer is cool, and over time can become completely devoid of oxygen, and of fish.
In stillwaters of more than a couple of metres in depth it pays to avoid fishing on the bottom in deep water in calm, hot conditions, as it is likely that the lake will have stratified, and the fish will be near to the surface, or in the margins.