Barometer to Measure Atmospheric Pressure

What is Barometric Pressure?

Barometric pressure can be described as the weight of the air at a specific location, measured in inches of Mercury (inHg) or hectopascals (hPa). The pressure at sea level is 29.92 inHg. Variations in barometric pressure are associated with changes in weather conditions. Falling pressure typically indicates precipitation can be expected, while rising pressure means weather conditions are improving.

Barometers are scientific instruments used in meteorology to measure atmospheric pressure. Aneroid barometers use a small, flexible metal box called an aneroid cell. This aneroid capsule or cell is made from an alloy of beryllium and copper. The evacuated capsule is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen.     

How do you read a Barometer?

It is advisable to lightly tap the glass near the center with your finger before taking a barometer reading. The mechanism is made deliberately "stiff" so that tapping the barometer will overcome any friction that may affect accurate readings, especially during periods of slow atmospheric changes. The ability to indicate changes in barometric pressure makes it a useful instrument for weather forecasting.

Pressure or Weather Trend

Subtle variations in barometric pressure greatly impact the weather. The weather forecast or pressure tendency is based on the rate of change of barometric pressure. In general, when the pressure increases, the weather improves (sunny to partly cloudy) and when the pressure decreases, the weather degrades (cloudy to rain). The weather forecast is an estimation or generalization of weather changes in the next 24 to 48 hours, and varies from location to location. The tendency is simply a tool for projecting weather conditions and is never to be relied upon as an accurate method to predict the weather. The barometer includes a manually set needle, which is used to mark the current measurement so a change can be seen. Barometric readings should be taken daily. Remember that the rate of change of barometric pressure is important in determining weather changes. You may want to take multiple readings each day during periods of unstable weather conditions.

How To Calibrate a Barometer?

Your barometer will operate from -100 to 3,800 feet with reliable accuracy. Aneroid barometers have a small screw on the back for sea-level calibration. With a flat blade screwdriver, turn this screw in either direction slightly while looking at the indicator needle. It should move in one direction or the other. Tap the barometer to see where the needle settles. Continue this procedure until the proper pressure reading is obtained. Do not turn the screw counter-clockwise (to the left) too far, since the screw can fall out. After the initial calibration, no further adjustment will be required unless the barometer is moved to a new geographic location.