Air pressure is the force exerted on a surface by the weight of the air molecules in the atmosphere above it, known as atmospheric or barometric pressure. Understanding this measurement is necessary because changes in the atmosphere affect everything from the weather we experience daily to the safety of air travel.
The Primary Tool
The instrument specifically designed to measure this atmospheric force is called the barometer. It provides a quantitative reading of the pressure exerted by the column of air above the instrument’s location. This measurement is often expressed in units like millibars (mbar) or inches of mercury (inHg). The initial concept was developed by Evangelista Torricelli in the mid-17th century, establishing that atmospheric pressure could be balanced and measured against a column of liquid.
How Barometers Function
Mercury Barometer
Torricelli’s original design evolved into the mercury barometer, which balances air pressure against a column of mercury. This device consists of a long glass tube, sealed at one end, and inverted into an open reservoir of mercury. The air pressing down on the reservoir forces the liquid up into the tube until the weight of the mercury column exactly balances the external atmospheric pressure. At sea level, this column height is approximately 760 millimeters.
Aneroid Barometer
A more modern and portable type is the aneroid barometer, which does not use any liquid. This instrument centers on a small, sealed metal chamber, often made from a beryllium and copper alloy, known as an aneroid cell. The air is largely pumped out of this flexible capsule, creating a near-vacuum inside. As external air pressure increases, the cell is slightly compressed, and as pressure drops, the cell expands. This movement is mechanically amplified through levers and springs to move a pointer across a calibrated dial.
Key Applications of Air Pressure Measurement
Weather Forecasting
Measuring air pressure is fundamental to meteorology, providing data for accurate weather forecasting. Changes in pressure are directly linked to large-scale weather systems. A rapid drop in barometric pressure indicates an approaching low-pressure system, associated with stormy weather and precipitation. Conversely, a rising or steady high-pressure reading signals the arrival of fair weather and clear skies. This ability to track pressure trends allows meteorologists to predict short-term changes in local conditions.
Altitude Determination
A second major application is the determination of altitude, utilized in aviation and mountaineering. Since air pressure naturally decreases as elevation increases, a device called an altimeter, which is essentially a specialized barometer, is used to measure height above sea level. Pilots and hikers rely on the predictable drop in atmospheric pressure corresponding to increased altitude. By converting the pressure reading using a known formula, the instrument displays the current altitude.