A barometer is an instrument designed to measure atmospheric pressure, which is the weight of the air above a given point. This measurement provides valuable information, particularly for understanding and predicting weather patterns. By observing changes in atmospheric pressure, individuals can gain insight into upcoming weather conditions.
Barometer Fundamentals
Most modern home barometers are aneroid barometers, which operate without liquid. They contain a small, sealed metal chamber from which most of the air has been removed. Changes in atmospheric pressure cause the chamber walls to expand or contract. These movements are then mechanically amplified and transmitted to a pointer on the barometer’s dial.
The dial displays pressure readings in common units such as inches of mercury (inHg) or millibars (mb), also known as hectopascals (hPa). A typical aneroid barometer dial features a scale ranging from about 28 to 31 inHg or 950 to 1050 mb/hPa. Many barometers include a set needle, a second movable needle that can be manually aligned with the main needle to track pressure changes over time. This allows for easy comparison of current pressure with previous readings.
Interpreting Barometer Readings
To accurately read an aneroid barometer, gently tap the glass face a few times. This helps overcome any slight friction in the internal mechanisms, allowing the main needle to settle at the precise current atmospheric pressure. Observe where the main needle points on the graduated scale, noting the numerical value.
Next, manually rotate the set needle until it precisely aligns with the main needle’s current position. This marks the starting point for future comparisons, allowing you to easily see whether the pressure has risen or fallen since your last observation. After a few hours, or the next day, repeat tapping and observing the main needle. If the main needle has moved relative to the set needle, it indicates a change in atmospheric pressure.
For initial setup or if you suspect your barometer is inaccurate, calibrate it by comparing its reading to a reliable local weather report. Many local weather stations or online sources provide current atmospheric pressure readings for your area. Adjust the small screw on the back of the barometer, usually with a tiny screwdriver, until its main needle matches the reported local pressure.
Using Pressure to Forecast Weather
The direction and rate of change in atmospheric pressure are more important for weather forecasting than the absolute reading itself. A rising barometer reading generally indicates fair weather is approaching or will continue. As pressure increases, it often signals the arrival of clear skies, stable conditions, and lighter winds due to denser, sinking air. A steady rise over several hours or a day typically suggests an extended period of good weather.
Conversely, a falling barometer reading often suggests unsettled or stormy weather is on its way. Decreasing pressure indicates less dense air is rising, which can lead to cloud formation, precipitation, and stronger winds. A rapid drop in pressure, especially over a short period, can be a precursor to severe weather such as thunderstorms or strong winds. A gradual fall, however, might indicate a prolonged period of rain or overcast conditions.
When the barometer reading remains steady, it generally means current weather conditions will persist. This indicates a stable atmospheric environment where no significant weather systems are moving in or out of the area. Observing these trends in pressure change provides a practical way to anticipate short-term weather shifts and plan activities accordingly.