The bottom of a freezer is typically colder than other parts due to the principle of convection and the cooling mechanism used in most modern freezers.
Convection: Convection is the process of heat transfer through the movement of fluids or gases. In a freezer, the air is continually circulating to maintain a consistent temperature throughout the freezer compartment. The cooling system usually directs cold air downward, which displaces warmer air upward. As the warmer air rises, it loses some of its heat to the freezer walls and other items stored in the freezer. This creates a temperature gradient, with the coldest air settling at the bottom of the freezer.
Cooling Mechanism: Most freezers use a cooling system that involves a refrigerant (often a gas) passing through a series of coils or tubes. The refrigerant absorbs heat from inside the freezer, causing it to evaporate and turn into a gas. This process cools down the surrounding air. The cold air, being denser, tends to settle at the bottom of the freezer.
As a result of these two factors, the bottom of the freezer is usually colder than the top or other parts of the freezer compartment. It's also worth noting that the temperature near the freezer walls may be slightly higher than the bottom due to the cooling effect being more concentrated in the center of the freezer. To ensure the most efficient and even cooling, it's a good practice to arrange items in the freezer with enough space for proper airflow and circulation.