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People predicted the weather back in ye good olde days before Doppler radar and GOES satellites using simple instruments. 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.
- glass, jar, or can
- plastic wrap
- a straw
- rubber band
- index card or lined notebook paper
Construct the Barometer
- Cover the top of your container with plastic wrap. You want to create an airtight seal and a 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.
- 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.
- 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.
- Record the location 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 the Barometer Works
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.
Predicting the Weather
Now that you have a barometer you can use it to help predict the weather. Weather patterns are associated with regions of high and low atmospheric pressure. Rising pressure is associated with dry, cool, and calm weather. Dropping pressure forecasts rain, wind, and storms.
- Quickly-rising pressure that starts from average or high pressure during fair weather indicates a low-pressure cell is approaching. You can expect the pressure to start to fall as poor weather approaches.
- Quickly rising pressure (over a few hours or a couple of days) after a period of low pressure means you can expect a short period of good weather.
- Slowly rising barometric pressure (over a week or so) indicates good weather that will stick around a while.
- Slowly falling pressure indicates the presence of a nearby low-pressure system. Changes in your weather are unlikely at this time.
- If the pressure continues to drop slowly you can expect a long period of bad (as opposed to sunny and clear) weather.
- A sudden drop in pressure (over a few hours) indicates an approaching storm (usually arriving within 5-6 hours). The storm probably involves wind and precipitation, but won't last long.