In winter, when the atmospheric temperature drops below 0°C, the surface water of a tank or ocean initially above 4°C starts losing heat to the air. As the surface water temperature falls below 4°C, it contracts and becomes denser, causing it to sink to the bottom. This process continues until the entire water mass reaches 4°C. As the top layers cool below 4°C, the water expands, reducing its density. Consequently, the water does not sink further but remains at the surface. When the air temperature falls below 0°C, the surface water gradually freezes into ice, while the water below the ice layer stays at 4°C. This is why the freezing of a water tank (or ocean) begins from the top and not the bottom.
