Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior.
Radiant heat moves through the air from warmer objects to cooler ones.
When radiation strikes an object, it is absorbed, reflected, or transmitted, depending on certain properties of that object. This is why solar-absorber surfaces tend to be dark colored. Bright-white materials or objects reflect 80 to 98 percent of incoming solar energy. Inside a home, infrared radiation occurs when warmed surfaces radiate heat towards cooler surfaces. For example, your body can radiate infrared heat to a cold surface, possibly causing you discomfort.