Date: March 2001
Can you please help me find a way to explain the
evaporation cycle to my second grade class?
For instance using simple terms and also a way to demonstrate it to them.
Isn't it amazing how difficult it is to explain
complex processes in simple terms?
The hydrologic cycle is shown as a figure in many
books, with clouds raining over land and water bodies,
evaporation occurring from the water body and
evapotranspiration from the land, and
streams/groundwater/surface runoff carrying water from
where it fell as rain or snow. I have such a diagram if you
would like me to fax it to you. You can arrange this
by asking the NEWTON coordinator to allow you to email me
The difficult part is explaining the change of state
between water and water vapor during evaporation (and
transpiration from plants) and between water vapor
in the air to clouds and then precipitation.
Evaporation occurs because there is less water in the air
than it can potentially hold, in simplistic terms. The more water vapor
there is in the air, the slower evaporation occurs (for a fixed
temperature and atmospheric pressure, which evaporation is also
Individual water molecules virtually escape from
a water surface or from a plant leaf into the air, aided often
by wind-caused turbulence (although molecular diffusion is the
main process). This could be demonstrated with balloons
filled with helium. From a box filled with the balloons
(the balloons held down with strings or a cover, which
would represent the water surface tension, a force that must be
overcome for an individual water molecule to escape), one
balloon could be released into a space above the box that is
the same size as the box. There is plenty of space for the
balloon to move into. As more balloons are released, there is
less room for each balloon and it is harder for it to find a
space to move into. When all balloons have been released, the
space above the box is filled; the air is saturated with water
molecules. If you put more balloons into the box and try to
release them, they have nowhere to go, because the air above
is already saturated (the space is already filled with balloons);
no more evaporation can take place.
You could use some variation on this to demonstrate
evaporation, the rate of evaporation, and the saturation of
air with water vapor.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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