Convection

Water naturally moving in a pot showing convection currents

Convection is the movement of heat because of the movement of warm matter. Convection is defined as the method of heat transfer through a fluid from places of higher temperature to lower temperature by the movement of the molecules due to their different densities.

For example, atmospheric circulation moves warm air to cool places, causing wind. Wind, in turn, can enter and cool a room if the window is open. The movement of the clouds, the ocean currents and many types of heaters are examples of convection.

Forced convection and natural convection

Convection can happen naturally ("natural convection") or because of a moving device ("forced convection").

The fan is a device that moves air artificially. The air in this case is moving because of the rotation of the fan. This is an example of "forced convection".

Natural convection happens because a fluid is lighter when hot and heavier when cold. Thus, if a fluid has a hot part and a cold part, the hot part naturally moves upward and the cold part naturally moves downward. For example, if the water in a pot is hotter near the bottom because of the fire, it rises from the bottom to the surface. At the same time the water near the surface is colder so it sinks to the bottom.

Convection currents

Convection currents occur when there are significant differences in temperature between two parts of a fluid. When this happens, hot fluids rise and cold fluids sink. This causes movements or currents in the fluid. Atmospheric circulation, for example, is made by convection currents.

ConvectionConvection Currents Media

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  Simulation of thermal convection in the Earth's mantle. Hot areas are shown in red, cold areas are shown in blue. A hot, less-dense material at the bottom moves upwards, and likewise, cold material from the top moves downwards.

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  Thermal image of a newly lit Ghillie kettle. The plume of hot air resulting from the convection current is visible.

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  Painting of William Prout, the chemist and physician

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  Fireplace, with grate and chimney

• Rayleigh–Bénard cells.

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  This color schlieren image reveals thermal convection originating from heat conduction from a human hand (in silhouette) to the surrounding still atmosphere, initially by diffusion from the hand to the surrounding air, and subsequently also as advection as the heat causes the air to start to move upwards.

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  Thermal circulation of air masses

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  Convection cells in a gravity field

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  Idealised depiction of the global circulation on Earth

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  Vector diagram of en:Foehn wind

Related pages

• Heat transfer
• Heat conduction
• Thermal radiation