Barometers measure atmospheric air pressure changes as they occur to forecast weather several hours in advance. Aneroid (nonliquid) barometers use incremental fluctuations in pressure upon the metal of a sealed evacuated (air free) drum or capsule to measure air pressure change. A faceplate and needle—similar to a clock face with hands—translates these changes mechanically into a readable measurement as the needle moves around the "face" of the barometer. Aneroid barometers use an "adjusting" screw on the back of the barometer to calibrate this needle to local pressure readings.
Check the manual or diagram that came with your barometer to confirm the location of the adjusting screw. If it did not come with a manual or diagram, check the back for a cover at the center or look for an open hole at the back of the barometer's mounting board that has a screw not attached to the board.
Confirm the current local weather and air pressure, either with a second previously calibrated barometer (if you own more than one) or on your TV, radio or an online weather site such as the Weather Channel that features hour-by-hour air pressure information by ZIP code. Type your ZIP code in the search engine and click the "Search" button; this will pull open current readings.
Match a screwdriver from your set against the screw head type.
Turn the screw slowly while checking the movement of the indication needle on the barometer's faceplate.
Tap the faceplate of the barometer lightly. Tapping the faceplate confirms that the needle has "settled" in the original spot you calibrated it to without loss in measurement accuracy. For example, if you tap the faceplate and the needle slides below your calibration point, slowly turn the screw to a point just above that area and then tap again to set the needle to the exact location.