by Wm. Robert Johnston
last updated 3 August 2002
The greenhouse effect is a relevant topic in classes of all levels, but most explanations in educational material give incorrect explanations. (Of primary and secondary textbooks I have encountered, over 90% have given incorrect and/or misleading explanations.) The atmospheric greenhouse effect is NOT the same process that heats a greenhouse, it does not involve reflection of infrared light or pollution, and it should not be confused with the global warming hypothesis (which is also generally misunderstood).
The demonstration below provides an analogy to the greenhouse effect avoiding the common misconceptions. It may be used to reinforce a physical explanation (such as that in "Global Warming" at http://www.johnstonsarchive.net/environment/wrjp365g.html).
Materials: open-topped transparent or translucent plastic container (quart-sized or larger for visibility); water source (faucet or hose); sink or bucket (to catch water)
Make a set of holes in the side of the container in a vertical line from top to bottom, with a spacing of about 2 cm. You may need to experiment to find the optimum size for the holes, which will be 2 mm or more, depending on your water source.
Procedure and explanation: Run water into the container, allowing it to fill part-way with water running out the side holes into the sink or bucket. The incoming water corresponds to incoming sunlight absorbed by the Earth, the outgoing water to thermal infrared radiation emitted by the Earth, and the level of water to the temperature of the Earth.
Note that the system rapidly attains equilibrium: the water level rises until water is escaping through enough holes that water leaves the container at the same rate it enters the container. The Earth emits thermal infrared radiation at a rate determined by its temperature.
The holes in the container correspond to the Earth’s atmosphere: the most abundant atmospheric gases (oxygen, nitrogen, argon) do not affect the emission of infrared light.
Now block one or two holes with your fingers. Notice that the water level rises, above additional holes until again water escapes at the same rate as water enters. Equilibrium is attained again, but with a higher water level than before.
The blocked holes represent the effect of greenhouse gases (water vapor and carbon dioxide, particularly). They prevent the emission of thermal infrared at some wavelengths, with the consequence that the Earth attains equilibrium at a high temperature than in the absence of any greenhouse effect.
© 2000, 2003 by Wm. Robert Johnston.
Last modified 3 August 2003.
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