Figure 3.18 Sources of heat gain and loss. The Sun is the original source of almost all heat. The heat from the Sun is exchanged among objects across the landscape. In the case of the cactus, heat is gained by direct sunlight, scattered sunlight, and reflected sunlight. Heat can be lost by evaporation of water vapor to the atmosphere. Heat can also be gained or lost by radiation from surrounding objects such as rocks, by conduction where the cactus comes into contact with the soil, and by convection as winds move hot or cold air over the surface of the cactus and disrupt its boundary layer.