Convecta

Meet Convecta, warm fluid that rises because she has become lighter than the fluid around her.

Heat a fluid and it usually expands, so its density falls: ρ = ρ₀/(1 + βΔT), where β is the volumetric expansion coefficient. Buoyancy then pushes the warmer, lighter fluid up, while cooler, denser fluid sinks to replace it. The result is a circulating flow, convection, which carries heat much faster than conduction alone.

For water near room temperature, β is about 2×10⁻⁴ per kelvin. Warming it by 10 K lowers its density by only about 0.2 percent, but that small difference is enough to drive rolling currents in a pot on a stove.

Convection shapes the weather, as warm air rises to form clouds and thunderstorms. It drives sea breezes, ocean circulation and the slow churning of Earth's mantle, and it carries energy through the outer layers of the Sun.

The simple density formula holds for small temperature changes. Water below 4 °C is an exception: it expands as it cools, which is why lakes freeze from the top down.