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Heat can spontaneously flow from a higher-temperature region to a lower-temperature region, but not the other way around. (The Second Law of Thermodynamics) We can see this in the diagram of the refrigerated trailer below which depicts the effect of heat from warm air outside penetrating and permeating from the trailer skin and the resulting internal airflows. The heat from warmer air outside flows to the cooler air-- warming it. As the forced chilled air moves across the top of the trailer from the nose to tail, the heat from the warm air spontaneously flows towards the cooler air. As the air returns along the base of the trailer, it continues to absorb heat from the floor and product, therefore the amount of cool air continues to decreases dramatically. The example stated above is under optimal conditions of unobstructed airflows in an empty, pre-cooled, sealed trailer. As the trailer is loaded with pallets of product, the flow of chilled air is obstructed; slowing the airflow and making less air absorb the same amount of heat and raising the air temperature in that vicinity. When warmer air infiltrates the trailer, this is made worse during the loading process.