In winter, when the atmospheric temperature drops below 0°C, the surface water of a pond cools and contracts. It continues to sink until the entire water mass reaches a temperature of 4°C. Further cooling causes the top layers to expand and become less dense, preventing them from sinking. When the air temperature falls below 0°C, the surface water loses heat and gradually freezes into ice. However, the water beneath the ice layer remains at 4°C. The layer of water in contact with the ice is at 0°C, while the layers below gradually increase in temperature up to 4°C. Fish can survive in the pond because the ice acts as an insulator, preventing heat transfer from the water to the atmosphere.
