Convection

Convection is the transfer of heat by the movement of a fluid, such as air, water or molten rock. Warmer fluid is generally less dense and rises, while cooler fluid sinks, creating circulating currents that carry energy from hot regions to cooler ones.

When a fluid is heated from below, the lower layer expands and becomes lighter than the layer above, so buoyancy pushes it up. When it cools at the top it sinks again, and a steady overturning pattern of cells develops. Heat transfer by convection is much faster than conduction in fluids, and it is one of the three modes of heat transfer along with conduction and radiation.

Sunlight heats the ground, which warms the air above it. The rising air cools, and water vapor condenses into cumulus clouds and thunderstorms, releasing latent heat that powers further lifting. The temperature drop with height is described by the lapse rate. On a global scale convection in the tropics drives the Hadley cell, and it carries heat and moisture upward, a major part of Earth's energy budget.

In the ocean, cooling and salt left behind by freezing make surface water sink in places, which drives the thermohaline circulation. Deep inside the planet, slow convection of the mantle moves the tectonic plates by a few centimeters a year, as described in plate tectonics. Convection is part of the more general topic of geophysical fluid dynamics.

• Hadley Cell — a global convection circulation
• Latent Heat — energy released in convective clouds
• Plate Tectonics — driven by convection in the mantle