Scientists Say: Convection

Convection (noun, “Kuhn-VEK-shun”)

Convection is one of three major ways heat moves from place to place. Heat, or thermal energy, naturally flows from hot to cold. It can get there through conduction, radiation or convection. Convection transfers heat through the movement of a liquid or gas.

If you’ve ever heated a pot of water on the stove, you’ve seen convection in action. First, water molecules near the heat source at the bottom warm up. That is, they gain thermal energy. These more energetic molecules speed up and spread out. That makes them less dense. As a result, they become more buoyant, or able to rise.

As those molecules rise, they give some of their thermal energy away to other, cooler molecules. In the process, those first molecules cool off, because they have lost some thermal energy. As they lose heat, they move more slowly and get closer together. They become denser. This causes them to sink back down to the bottom of the pot, where they reheat and rise again. This loop of rising and falling water is a convection current. It gradually warms water throughout the pot, even though the only heat source is at the bottom.

Convection is behind many natural phenomena. The churning of warm and cool air in the atmosphere creates wind. Rising warm water and falling cool water drive ocean currents. And the convection of molten rock deep inside Earth is thought to shuffle around the tectonic plates on its surface.

In a sentence

Convection may help explain the Mpemba effect, which happens when hot water freezes faster than cold water.

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