A barometer measures atmospheric pressure, which is the weight of the air column above a specific location. Monitoring changes in this pressure provides a simple yet effective method for forecasting short-term weather shifts; generally, rising pressure indicates fair weather, while falling pressure suggests approaching storms. To function correctly as a predictive tool, the device must first be accurately set to reflect the current atmospheric conditions in its specific geographic location. This process, known as calibration, requires matching the instrument’s reading to a verified local pressure reference point. A barometer that is not properly calibrated offers meaningless readings.
Locating the Correct Pressure Reading
Before physically adjusting the barometer, obtain the correct reference pressure for your current location. This number is readily available from reliable sources like local airport weather stations, government meteorological services (such as NOAA), or trusted online weather applications. When seeking this data, understand the difference between station pressure and sea-level adjusted pressure.
Station pressure is the raw, actual atmospheric pressure measured at the instrument’s specific elevation. Because atmospheric pressure naturally decreases as elevation increases, station pressure varies significantly between nearby towns located at different heights. This variance makes direct comparison difficult for forecasting across regions.
The industry standard for weather reporting and barometer calibration is the sea-level adjusted pressure, often called the altimeter setting. This reading mathematically accounts for the altitude of the measuring station, adjusting the pressure value to what it would theoretically be at sea level. Using this standardized figure ensures that your barometer’s reading is comparable to those reported by nearby weather services, making it the appropriate value for setting your device.
Calibrating an Analog Barometer
The physical calibration of a standard analog aneroid barometer focuses on aligning the internal mechanism with the verified sea-level adjusted pressure reading. Most aneroid barometers feature a small adjustment screw located on the back or sometimes on the side of the casing. This screw is directly connected to the internal bellows mechanism that responds to pressure changes.
To begin the process, gently tap the glass face of the barometer a couple of times. This action overcomes minor friction within the delicate mechanical linkages, ensuring the internal indicator needle settles accurately to the current pressure reading.
Insert a small, fine-tipped screwdriver—often a jeweler’s flat-head—into the adjustment screw slot. While keeping the instrument steady, turn the screw slowly and deliberately, observing the movement of the fixed indicator needle on the dial. The goal is to rotate this needle until it precisely points to the sea-level adjusted pressure value you obtained from the weather service.
The adjustment requires patience, as the screw usually offers fine control, meaning small turns result in noticeable needle movement. It is advisable to make tiny, incremental turns, checking the needle position after each movement to avoid overshooting the target pressure. Once the fixed needle is set to the correct reference value, the instrument is calibrated to your local atmospheric conditions.
The final step involves the movable pointer, often a brass hand located on the outside of the glass. This hand is not connected to the internal mechanism but serves as a marker. Manually rotate this external pointer to perfectly align with the newly set fixed indicator needle. This marker will later be used to track the direction and magnitude of pressure changes over time, aiding in weather prediction.
Ensuring Long-Term Accuracy
Initial calibration sets the baseline, but maintaining accuracy requires periodic attention and correct usage. Before taking any reading, make it a habit to gently tap the glass face, repeating the action performed during the initial setup. This ensures the internal needle is not stuck and accurately reflects the current pressure, overcoming any minor static friction that may develop in the mechanism.
The external, manually set indicator hand is used to track pressure changes relative to the previous reading. By aligning the brass pointer with the fixed needle after each reading, you can easily observe if the pressure has risen (needle moves past the marker) or fallen (needle lags behind the marker). Barometers should be checked against the official local reference pressure approximately once a month, or immediately if the device is moved significantly, as changes in temperature or altitude can slightly affect the calibration over extended periods.