Kids, it's easy to make your own weather barometer! Using simple instruments, people predicted weather back in the good ole days before Doppler radar and GOES satellites. One of the most useful instruments is a barometer, which measures air pressure or barometric pressure. You can make your own barometer using everyday materials and then try to forecast the weather yourself.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

You’ll need a glass, jar, or can, plastic wrap, a straw, a rubber band, an index card or lined notebook paper, tape, and scissors. To make the barometer:

1. Cover the top of the container with plastic wrap making an airtight seal and smooth surface. Secure the plastic wrap with a rubber band. The most important part of making the barometer is getting a good seal around the rim of the container.
2. Lay the straw over the top of the wrapped container so that about two-thirds of the straw is over the opening. Secure the straw with a piece of tape.
3. Either tape an index card to the back of the container or else set up your barometer with a sheet of notebook paper behind it.
4. Mark the location of the end of the straw on your card or paper. Over time the straw will move up and down in response to changes in air pressure. Watch the movement of the straw and record the new readings.

How does it work?  High atmospheric pressure pushes on the plastic wrap, causing it to cave in. The plastic and the taped section of straw sink, causing the end of the straw to tilt up. When atmospheric pressure is low, the pressure of the air inside the can is higher. The plastic wrap bulges out, raising the taped end of the straw. The edge of the straw falls until it comes to rest against the rim of the container. Temperature also affects atmospheric pressure so your barometer needs a constant temperature in order to be accurate. Keep it away from a window or other places that experience temperature changes.

So how do you predict the weather? Weather patterns have regions of high and low atmospheric pressure. Rising pressure means dry, cool, and calm weather. Dropping pressure forecasts rain, wind, and storms.

• Quickly rising pressure (over a few hours or a couple of days) after a period of low pressure means you can expect some good weather.
• Slowly rising barometric pressure (over a week or so) indicates good weather that will stick around a while.
• Quickly rising pressure that starts from average pressure during fair weather indicates a low pressure cell is approaching. You can expect the pressure to start to fall as poor weather approaches.
• Slowly falling pressure indicates the presence of a nearby low pressure system and changes in weather are unlikely.
• If the pressure continues to drop slowly you can expect a long period of bad weather.
• A sudden drop in pressure (over a few hours) indicates an approaching storm within 5-6 hours. The storm probably involves wind and precipitation but won't last long. 

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
November 2008

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Reference:  Anne Marie Helmenstine at: http://chemistry.about.com/b/2008/09/06/make-a-simple-weather-barometer.htm?nl=1