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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