Convection is the process in which heat transfer occurs due to the motion of fluid from one area to another. It can be classified into two categories, free convection and forced convection. In free convection, heat transfer takes place by the flow of fluid due to the difference in densities caused by a temperature gradient. In forced convection, an external source, like a pump or fan, is used to flow the fluid for heat transfer. Another heat transfer mechanism is radiation, for which no medium is required. In radiation, a hot object releases electromagnetic waves like ultraviolet, visible, or infrared, which are absorbed by a colder body. The rate of heat transfer through radiation is proportional to the body's surface area and the fourth power of the absolute temperature. It also depends on the emissivity of the radiating body. This relationship is called the Stefan-Boltzmann law of radiation, where sigma is the Stefan-Boltzmann constant. Its numerical value is 5.67 × 10−8 joules per second per square meter per kelvin to the power four.