When water at 0℃ is heated to 4℃, its volume not only does not increase, but decreases. When the temperature of water is higher than 4℃, its volume will expand with the increase of temperature. So water has the smallest volume and the largest density at 4℃.
On the surface of the water in the lake, when the temperature drops in winter, if the water temperature is above 4℃, the water in the upper layer will cool, the volume will decrease, the density will increase, and it will sink to the bottom, while the warm water in the lower layer will rise to the upper layer.
The cold water in the upper layer constantly exchanges positions with the warm water in the lower layer, and the overall water temperature gradually decreases. This thermal convection phenomenon can only be carried out until the temperature of all water reaches 4℃.
When the water temperature drops below 4℃, the water in the upper layer expands instead, and the density decreases, so the cold water layer stays on it and continues to cool until the temperature drops to 0℃, and the cold water layer on it forms ice. The main form of heat exchange in the above stage is convection.
When the water surface freezes, the cooling of water depends entirely on the heat conduction of water for heat transfer. Due to the poor thermal conductivity of water, the water temperature at the bottom of the lake is still around 4℃. Only water in the range of 0℃ to 4℃ shows abnormal expansion.