How to Forecast With a Barometer

Set your barometer to current atmospheric pressure, unless using a homemade barometer. You can find out the current reading by checking the weather station on TV, calling your local airport or searching the National Weather Service (NWS) website for your current location. If using an aneroid barometer, lightly tap the glass, then turn the adjustment screw on the back to set the reading pointer to the current pressure. If using an electronic barometer, press the reset button, then press the calibration button and enter the current pressure. You may also need to set the current altitude, which can be obtained by calling your local airport.

Mark or note the current air pressure. If using a homemade barometer, simply mark a line on the bottle to indicate the water level. If using an aneroid barometer, write down the measurement or turn the knob on the front of the device to move the marker arm so it covers the reading pointer. If using an electronic barometer, simply write down the measurement.

Track the new atmospheric pressure after 24 hours. In an aneroid barometer, the reading pointer may move left or right from the marker arm, indicating a decrease or increase in atmospheric pressure, respectively. With a homemade barometer, water level may be higher or lower than the original mark. A digital barometer will clearly display the change in pressure. Mark or write down the new air pressure level.

Analyze the difference in air pressure levels. Rising air pressure is a sign of good weather, since winds blow toward areas with lower pressure. For the same reason, falling air pressure signals poor weather. A great difference between the first and second measurements indicates a more dramatic change in weather. If the air pressure drops one millibar per hour or more, you can expect stormy weather.

Record the air pressure and current observed weather in a neat chart every day. This will help you predict the weather more accurately for your area.