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Educator's Guide to Convection

Courtesy of the Jet Propulsion Laboratory


Images and Animations Illustrating Convection

Courtesy of Andrea Malagoli


Capturing Convection

Sometimes a single physical process in nature can explain a variety of events. Convection is one such process. It functions because heated fluids, due to their lower density, rise and cooled fluids fall. A heated fluid will rise to the top of a column, radiate heat away and then fall to be re-heated, rise and so on. Gasses, like our atmosphere, are fluids, too. A packet of fluid can become trapped in this cycle. When it does, it becomes part of a convection cell.

Convection cells can form at all scales. They can be millimeters across or larger than Earth. They all work the same way. The convection that students are most likely to have observed is in cumulonimbus clouds or "thunderheads." These towering vertical clouds can be seen to evolve over a few minutes. The tops of the clouds have a sort of cauliflower appearance as warm moist air rises through the center of the cloud. The moisture in the cloud condenses as it cools. The air gives up some of its heat to the cold high altitude air and begins to fall.

As the air falls along the exterior of the cloud, it returns to warmer low altitudes where it can be caught up in the rising column of air in the center of the cloud. This fountain-like cell can form alongside other cells, and a packet can move between cells. Hail forms when water droplets, carried by the strong updrafts, freeze, fall through the cloud and are caught in the updraft again. An additional layer of water freezes around the ice ball each time it makes a trip up through the cloud. Eventually, the hail becomes too heavy to be carried up anymore, so it falls to the ground. Large hailstones, when cut apart, show multiple layers, indicating the number of vertical trips the stone made while it was caught in the convection cell.

Convection also occurs on the Sun. A high resolution white light image of the Sun shows a pattern that looks something like rice grains. Very large convection cells cause this granulation. The bright center of each cell is the top of a rising column of hot gas. The dark edges of each grain are the cooled gas beginning its descent to be re-heated. These granules are the size of Earth and larger. They constantly evolve and change.

Thunderheads and granulation are large-scale examples of convection. Fortunately, there are examples of convection that fit into a classroom. An excellent example can be seen in hot Japanese Miso (soybean paste) soup.

The interior of the broth is hot. The surface of the soup is exposed to cool air. Hot packets of fluid rise out of the interior of the soup to the surface where they give off heat. Now cooled, they fall down into the bowl to be re-heated. Left alone, the soup will dissipate its heat in this way (and through conduction with the sides of the bowl) and reach room temperature.

The soybean paste granules and other ingredients will highlight the convection cells vividly. As students gaze into their soup, they will see the rising and descending columns of fluid. The cells will evolve and change their positions. Dark bottomed bowls show the effect best. If the soup is stirred up, students can observe the cells reform. Of course, the demonstration material can be consumed at the conclusion of the demonstration.

Convection acts as described in the examples above where gravity's effects are present (so that warm, low density fluids can rise and cool, high density fluids can fall). What happens in the weightlessness of space where up (rise) and down (fall) have no meaning?


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