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With all the talk about global warming these days, do you know how to
explain the greenhouse effect to your kids? For a great example of this
phenomenon, look no further than your own driveway!
Did you know...
You can observe all the principles of the greenhouse effect in a parked
car in the sun:
- Sunshine (solar energy) passes easily through the glass to heat objects in
the car's interior -- remember how hot the car seat gets in summer?
- The car's interior absorbs the short-wave energy and heats up. When the
seats heat up, they produce long-wave infrared radiation.
- Here's the tricky
part: The glass in the car's windows now begins to act as a kind of one-way
mirror. Short-wave solar energy continues to enter with no problem but much
of the long-wave infrared radiation is blocked and prevented from leaving.
On a much larger scale, this is what's happening to the earth:
- Energy from the sun hits the earth's atmosphere as solar radiation. Some
of it is bounced back into space by the atmosphere, but most passes through
the atmosphere to warm the surface of the earth.
- Once the earth has been warmed by the short-wave solar energy, excess heat
is radiated back into the environment as long-wave infrared radiation.
- Some of the gases in earth's atmosphere act like the glass in the car
windows. They let in solar energy and block or absorb infrared energy. As a
result, the atmosphere gets warmer.
What does pollution have to do with it?
In all, 30 greenhouse gases have been discovered to date, including carbon
dioxide (CO2), water vapor, methane and ozone. But lately new gases are being added to the mix:
Chlorofluorocarbons (CFCs). These are the harmful gases produced by cars
and factories, and we humans are responsible!
A simple greenhouse experiment you can try at home:
- Line a large open bowl with dark cloth or paper.
- Place the bowl in the sun and put an inverted paper cup in the bowl. Lay a
thermometer across the top of the cup so that you are measuring the air
temperature in the bowl.
- Note the temperature.
- Cover the bowl with a sheet of clear plastic wrap. Note the new
temperature reading. The increase in air temperature is due to the trapped
heat.
NOTE: Be careful, it can get rather hot. You'll want to keep an eye on the
temperature reading so the thermometer doesn't pop its top. When I tried
this experiment with a simple homemade solar collector, I recorded interior
temperatures approaching 200° F. Whew!
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