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Convective heat transfer, often referred to simply as convection, is the transfer of heat from one place to another by the movement of fluids. Convection is usually the dominant form of heat transfer in liquids and gases. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of unknown conduction and advection.
Because of convection! There are three types of heat transfer: conduction, convection and radiation. Convection is a type of heat transfer that can only happen in liquids and gases, because it involves those liquids or gases physically moving. Convection happens when there is a difference in temperature between two parts of a liquid or gas. The hot part of a fluid rises, and the cooler part sinks. But let's use an example to think about why it happens, lest we assume that the fluid has a mind of its own.