Atmospheric pressure is the force exerted on a surface by the weight of the air column above it. This pressure constantly fluctuates due to changes in temperature and weather patterns, making its precise measurement a necessity for science and industry. The instrument designed specifically to quantify this force is the barometer, which literally means “weight measure.” Barometers come in several forms, each based on a distinct physical principle to translate atmospheric weight into a readable value.
The Principle of Liquid Barometers
The earliest instrument for measuring air pressure was the liquid barometer, developed in the 17th century by Evangelista Torricelli. This device, often called a Torricellian barometer, works by balancing the weight of the atmosphere against a column of liquid. The classic design involves inverting a long glass tube, sealed and filled with a dense liquid like mercury, into a dish of the same liquid.
The atmosphere pushes down on the exposed surface of the mercury in the dish, preventing the liquid inside the inverted tube from completely draining. The column falls until the pressure it exerts equals the external atmospheric pressure, and its height directly indicates the current air pressure.
A standard atmosphere at sea level supports a column of mercury approximately 760 millimeters high, creating the Torricellian vacuum above the liquid. Fluctuations in atmospheric weight cause the column to rise and fall, offering a direct visual measurement of pressure changes.
The Principle of Mechanical Barometers
Modern measurement often relies on the mechanical barometer, known as the aneroid type, which operates entirely without liquid. The central component of this device is a small, flexible metal chamber called an aneroid cell. This cell is sealed and partially evacuated to create a near-vacuum inside.
As external atmospheric pressure increases, the cell is compressed, causing its walls to contract. When the pressure drops, the cell expands outward due to the minimal internal pressure. This minute movement is then translated and amplified through a system of mechanical linkages, springs, and levers.
These linkages move a pointer across a calibrated dial, providing a visible reading of the pressure change without the hazards of liquid mercury. Digital barometers use similar principles, often employing micro-electromechanical systems (MEMS) sensors. These sensors use sensitive diaphragms where pressure-induced deformation is translated into an electrical signal, allowing for precise electronic data logging.
Standard Units of Air Pressure Measurement
Air pressure is expressed using several standard units globally. One common unit, derived directly from the liquid barometer, is Inches of Mercury (inHg). This unit is frequently used in aviation and by weather services in the United States, representing the height of the mercury column.
Another widely used meteorological unit is the Millibar (mb), which is numerically equivalent to the Hectopascal (hPa). These units are the international standard for weather reporting, with an average sea-level pressure being around 1013.25 hPa. Millibars or hectopascals are preferred in most scientific and meteorological communities.
A third unit is Pounds per Square Inch (psi), which is an engineering unit representing a force applied over a specific area. While psi is not used for large-scale atmospheric weather reports, it is commonly encountered for measuring pressures in tires or compressed air tanks.
Practical Uses of Air Pressure Data
The data collected from barometers is applied across several fields, most significantly in weather forecasting. Changes in atmospheric pressure are directly linked to the movement of air masses and the development of weather systems. A rapidly falling pressure reading often signals the approach of a low-pressure system, which is associated with unstable, stormy weather and precipitation.
Conversely, rising or consistently high pressure usually indicates a high-pressure system, which often brings clear skies and stable weather. Meteorologists rely on these pressure tendencies to track and predict the intensity and path of storms. Pressure data is also fundamental to altimetry, which is the measurement of altitude.
Since air pressure naturally decreases predictably as altitude increases, a barometer can be calibrated to function as an altimeter. This application is crucial for aviation, where aircraft altimeters measure height above sea level by sensing the change in surrounding air pressure. Hikers and climbers also use portable barometric altimeters to track their elevation gain.