Convection: Heat on the Move in Liquids and Gases

Why is the top bunk warmer?

In many homes, the upstairs is warmer than the basement, and the top bunk is warmer than the bottom one. Heaters are often placed low, under windows. All of this is explained by convection.

How convection works

In conduction, energy passes from particle to particle while the particles stay in place. In convection, the warm particles themselves move and carry their energy with them. That can only happen in fluids (liquids and gases), where particles are free to move around. In a solid, the particles are held in place, so there is no convection.

1. A fluid is heated from below. Its particles move faster and spread apart, so it expands and becomes less dense.
2. The less dense, warmer fluid rises. Cooler, denser fluid sinks to take its place.
3. The rising fluid moves away from the heat, cools, becomes denser and sinks again.
4. The sinking fluid is heated again, and the loop repeats. This loop is a convection current.

Convection at home

• A baseboard heater or radiator warms the air beside it. The warm air rises to the ceiling, spreads across the room, cools and sinks, then flows back to the heater. The whole room warms up.
• Heaters are often put under windows, where cold air sinks. The heater warms that air before it can spread along the floor.
• Warm air collects near the ceiling and upstairs; cool air settles near the floor and in the basement.
• Cool air from an air conditioner sinks and spreads along the floor, then warms and rises, so the whole room slowly cools.

Convection in nature

• Wind: the Sun heats Earth's surface unevenly. Air over warmer land rises, and cooler air flows in to take its place. That moving air is wind.
• Lake breezes: on a sunny summer day, land heats up faster than water. Air over the land rises, and cooler air from over the lake flows in toward the shore. That is why a beach on Lake Huron or Lake Ontario is often cooler and breezier in the afternoon. At night the pattern can reverse, as land cools faster than water.
• Thunderstorms: on hot, humid days, warm moist air rises quickly. As it rises, it cools and the water vapour condenses into tall clouds that can grow into thunderstorms.
• Soaring birds: hawks and turkey vultures circle upward without flapping by riding thermals, columns of rising warm air.
• Lake turnover: in fall, the surface water of an Ontario lake cools, becomes denser and sinks, mixing the whole lake. In spring, after the ice melts, the surface water warms toward 4 °C (when water is densest) and the lake mixes again. Turnover carries oxygen down to fish and nutrients up toward the surface.
• Ocean currents: cold, dense water sinking in polar seas helps drive slow currents that carry heat around the world's oceans.
• Inside Earth: hot rock deep in the mantle flows extremely slowly, a few centimetres a year, in convection currents. Scientists think these currents help move Earth's tectonic plates.