Air has weight and exerts a pressure called air pressure. When a barometer is used to measure air pressure, this pressure is called barometric pressure. An aneroid barometer is the most common type of barometer. It employs a thin-walled metal chamber that expands and contracts with changes in atmospheric pressure, causing an attached needle to move to indicate the barometric pressure. Aneroid barometers need to be adjusted from time to time and can be set with a screw on the back of the case.
Find the correct barometric pressure. Visit the Aviation Weather Center site (see Resources), navigate to "Local forecast by 'City, St' city, state or Postcode" and enter the information for your location.
You can also use a weather radio and tune it to the NOAA weather report for your location. NOAA weather radio broadcasts give detailed forecasts as well as the current local barometric pressure.
Convert the barometric pressure to inches of mercury if it is given in millibars. Multiply millibars by 0.0295. For example: 1013 millibars X 0.0295 = 29.9 inches of mercury.
Use the screwdriver to turn the adjustment screw on the back of your barometer to set it to the correct barometric pressure. Usually only a slight turn is necessary.
To forecast the weather, it is not necessary to set your barometer to the exact barometric pressure. What is important is the rising or falling of the barometric pressure. Usually a rapidly falling barometer indicates stormy or windy weather and a rising barometer forecasts fair weather.
Treat a barometer gently, as it is a delicate device and is easily damaged.