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Free convection, also known as natural convection, is a phenomenon which occurs when a fluid or gas moves as a result of density changes which occur inside it, rather than in response to an external source of movement such as a fan or turbine. Natural convection can be seen, unsurprisingly, in nature, where it plays a role in a number of different natural processes, and it can also be seen in human-controlled settings, ranging from the kitchen to the chemistry lab. This differs from forced convection, in which movement is forced by the movement of a device such as a fan, as seen in a convection oven.
Natural convection occurs when fluids become unstable. One area becomes warmer than the rest, losing density, and it starts to rise, creating a small current. As it rises, heat is transferred to the surrounding cooler fluid. Currents and eddies appear. In a simple example of natural convection, if one drops hot water dyed with food coloring into a clear glass bowl of undyed cold water, it will be possible to trace the movement of the dyed water until the mixture stabilizes and the movement stops.
Newton's Law of Cooling plays a critical role in free convection. According to Newton, the rate of change temperature-wise within a fluid can be viewed as proportional to the difference between the fluid and the surroundings. This influences free convection, because changing temperatures create instability, which leads to free convection. For example, weather systems are heavily influenced by the movement of hot and cold air which are influenced by solar radiation, contact with cold water, and so forth.
The circulation of the world's oceans is a direct consequence of free convection. As water moves towards the poles, it cools and becomes denser, sinking toward the bottom. Meanwhile, water moving towards the equator warms up and becomes lighter, rising to the surface. This creates a series of complex currents throughout the ocean which generate continuous circulation, keeping the water in the ocean in constant motion.
Convection theory also explains other natural processes, such as the movement of fluids beneath the Earth's crust. The shift of liquids beneath our feet plays a role in a number of natural processes, with these fluids constantly moving as a result of temperature changes inside the Earth. Convection currents can also intersect with each other, as seen in the interaction between the ocean and weather systems over and near the ocean which are influenced by temperature changes caused by the movement of ocean currents.