Horizontal motion of fluid within a larger layer of the same fluid in a gravity field.

Advection develops in response to areas of different pressure at the top and bottom of the layer; these areas are the tops and bottoms of convection columns (vertically moving fluid) within the layer, driven by heat from an outside source. Columns of warmer, lighter fluid ascend, creating areas of relatively low pressure at the bottom and relatively high pressure at the top; similarly, columns of cooler, heavier fluid descend, creating areas of relatively high pressure at the bottom and lower pressure at the top. In response, horizontal advection currents develop, moving from the high pressure areas towards the low pressure areas. Eventually conveyor belts of fluid (convection cells) circulate through the layer, transferring heat from the bottom to the top.

Although convection is a more familiar word, the effects of advection are more familiar in everyday life:

  • Advection in the Earth's atmosphere is better known as wind.
  • Advection in the Earth's oceans is usually called a "current", either a surface current like the Gulf Stream, or a deep sea current that flows in the other direction.
  • Advection at the top of the Earth's mantle drags pieces of the crust along with it, driving plate tectonics. The advecting mantle plus the crust is known as the lithosphere.

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