Convection is a type of transport (of matter and energy), absent in solids and negligible for very viscous fluids, caused by a pressure gradient and the force of gravity and characterized by circulation motions inside the fluid.
The resulting convective motion is a state of motion characterized by a high degree of mixing, which depends on the hydrodynamic regime: in particular, in the laminar regime the degree of mixing is lower, while in turbulent regime the degree of mixing is greater, and consequently, the exchange coefficients are higher.
In thermodynamics, thermal convection is the heat transfer due to bulk movement of molecules within fluids such as gases and liquids, including molten rock (rheid).
The phenomenon of thermal convection occurs when a fluid (such as water or air) comes into contact with a body whose temperature is higher than that of the fluid itself. By increasing the temperature by conduction, the fluid in contact with the object expands and decreases in density, and due to the thrust of Archimedes it rises being less dense than the fluid that surrounds it, which is colder, thus generating convective motions, in which the fluid warm rises to the top and the cold drops down (natural convection). Convection takes place through advection, conduction, or both.