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Name: Paula
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: March 2001


Question:
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.



Replies:
Paula,

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 directly.

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 dependent on).

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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