Convection: Heat Through Movement

Boil a pot of water and watch closely. You'll see the water circulating — rising in the middle, sinking at the edges.

This is convection: heat transfer through the bulk movement of fluid (liquid or gas).

When water at the bottom absorbs heat, it expands slightly and becomes less dense. Less dense things float — just like a hot air balloon. The warm water rises. Meanwhile, cooler, denser water at the top sinks to take its place. This creates a loop called a convection cell.

People say "heat rises." That's not quite right. Hot fluid rises because it's less dense. Heat itself doesn't have a direction. In space, without gravity, there's no convection at all — there's nothing to make the less-dense fluid float.