What is a good junior high science experiment on the
topic of dewpoint?
I'm sorry that it took me a while to respond to your question.
One popular experiment is to take a metal container that is shiny on the
outside and place it on something printed or patterned (like newsprint)
that can be reflected from the side of the container.
Place a thermometer into the container and then mostly fill it with room
temperature water. Slowly add ice and slowly stir the mixture; keep doing
this until the printing or pattern is not clearly
reflected from the outside of the container (dew has formed on the outside
of the container). Read the thermometer - the temperature is very close
to the dewpoint (it may be just under the dewpoint because of the lag of
heat transfer through the container).
You can try this both inside the building and outside (outside if it is
above freezing that is) to see the difference in the dewpoint inside and
outside. Since there is usually more water vapor in the building (if not
air conditioned), the dewpoint temperature should be higher. If the
building is air conditioned, the dewpoint temperature outside is usually
higher. The dewpoint temperature reflects the absolute amount of water
vapor in the air (as opposed to relative humidity, which is temperature
David R. Cook
Environmental Research Division
Argonne National Laboratory
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Update: June 2012