How to Explain Heat Transfer in a Convection Cell on Earth

Place both the ice cube and the rock in the glass of water. Point out to your audience that one of the two items floats while the other sinks, even though the ice cube weighs more and is bigger than the rock.

Explain to your audience that density is what determines whether an item floats or sinks -- its density as compared with the density of the surrounding fluid. The rock is more dense than the water, so it sinks. The ice is less dense than the water, so it floats.

Explain that fluids expand when they are heated. With liquids like water, this expansion is usually very slight; with a gas, by contrast, it is significant. In any case, however, when they are heated, both liquids and gases become less dense.

Ask your audience to think about water heating in a saucepan. The water near the bottom is absorbing all the heat, so it's becoming less dense and will start to rise, while the colder denser water above it will start to sink. The result will be a circular current, where hot water is rising in some areas and cold water is sinking in others.

Draw a saucepan on a whiteboard or chalkboard, and draw arrows running in a circle inside it to show your audience the direction of flow inside the saucepan. Explain that these same kinds of convection currents take place in Earth's atmosphere, only on a far greater scale.

Give a couple examples to illustrate. One of the most important is atmospheric circulation cells. The Earth receives more sunshine at the Equator than at higher or lower latitudes, so rising hot air carries heat North and South, while sinking cool air flows back towards the Equator. Another "loop" created by uneven heating operates between 30 and 60 degrees North and South latitude, and a final pair of loops operates near the Poles. These convection cells transfer heat on a gigantic scale, and they are very important for currents and weather patterns.

Offer plate tectonics as another example of convection cells in action. Gigantic convection cells in the liquid rock of Earth's mantle carry heat from the core toward the surface. These slow but inexorable currents push on the plates that form Earth's crust, causing the motion of the plates that leads to phenomena such as earthquakes and seafloor spreading. Like atmospheric circulation cells, these currents in the mantle are another example of convection cells important to our daily lives.