A barometer is a scientific instrument designed to measure atmospheric pressure, providing a quantifiable reading of the force exerted by the air above a specific point. Invented by Evangelista Torricelli in the mid-17th century, the barometer marked a significant advance in physics and meteorology. It provided the first means to systematically observe and record changes in the air that constantly surrounds us.
Understanding Atmospheric Pressure
Atmospheric pressure is the weight of the entire column of air extending from the top of the atmosphere down to the measurement point. This air column exerts considerable force on everything at the Earth’s surface.
Pressure fluctuates due to changes in air density, which are driven by temperature and the movement of air masses. Warm air is less dense and creates lower pressure, while cold air is denser and results in higher pressure. These differences in pressure create the large-scale weather systems that move across the globe.
Standard sea level pressure serves as a baseline, typically measured at 1013.25 millibars (mb) or hectopascals (hPa). In the United States, pressure is also commonly reported in inches of mercury (inHg), where the standard sea level value is 29.92 inHg. The millibar is the preferred measurement for modern meteorological reporting.
Even minor variations from the standard sea level value signify a shift in the atmosphere’s condition. Measuring these small changes allows for an understanding of the dynamic system of the air above us. The continuous observation of these pressure shifts forms the foundation of weather prediction.
Construction and Mechanics of Barometers
The mercury barometer was the first type, developed around 1644. This instrument consists of a long glass tube, sealed at one end and inverted over a dish of mercury. Atmospheric pressure pushes down on the mercury in the dish, forcing a column of mercury up into the vacuum-sealed tube until the weight of the column balances the external atmospheric force.
The height of the mercury column, typically around 760 millimeters at sea level, directly indicates the pressure. As atmospheric pressure increases, the column rises, and as it decreases, the column falls. Because mercury is toxic, this liquid barometer has been largely replaced for general use by the aneroid barometer.
The aneroid barometer operates without liquid, relying instead on a small, flexible metal capsule called an aneroid cell. This cell has most of the air pumped out, creating a near-vacuum within its walls. Changes in external air pressure cause the thin metal walls of the capsule to expand or contract slightly.
A system of mechanical levers and springs is attached to the capsule to amplify these minute movements. This linkage moves a pointer across a calibrated dial, translating the physical expansion or contraction into a pressure reading. Modern digital barometers use micro-electromechanical systems (MEMS) to measure the deflection of a tiny diaphragm electronically, offering a compact and precise alternative.
Interpreting Barometric Readings
The application of a barometer lies in interpreting the trend of its readings rather than just the absolute value at a single moment. Rising pressure indicates the approach of a high-pressure system, which is associated with sinking air and fair, stable weather. A steady or slow rise suggests that the current good weather will continue.
Conversely, falling pressure signals the approach of a low-pressure system, where air is rising and often leads to cloud formation, wind, and precipitation. A gradual drop in pressure may suggest light rain or overcast conditions, but a rapid or significant drop often precedes severe weather, such as a storm or frontal system. Monitoring the rate of change is essential for short-term weather forecasting.
Barometers are also used as altimeters, particularly in aviation and hiking, because air pressure decreases predictably as altitude increases. At higher elevations, there is less air above, resulting in a lower reading. The instrument can be calibrated to translate this pressure reading directly into an approximate altitude above sea level.