In these images, we can see how a leak in the return air duct/air handler enclosure leads to hot attic air being "pulled" (via suction from the return side of the air handler) down through the wall cavity. This adds to the overall thermal load for the cooling system, and is easily fixed by sealing the return duct/air handler enclosure. In this case, the air is being pulled down through the communication of the air handler enclosure wall with the attic. The negative pressure comes from leakage into the metal air handler cabinet rather than the duct itself.
The temperature range is from 71°F (darkest colors) to 83°F (lightest colors).