A barometer is a scientific instrument designed to measure atmospheric pressure, or air pressure, primarily for weather forecasting. By tracking the changes in air pressure over time, meteorologists and hobbyists can predict short-term changes in local weather patterns. The device provides a direct reading of the air’s weight, which indicates shifting weather systems.
Understanding Atmospheric Pressure
Atmospheric pressure represents the force exerted by the weight of the air column extending from the measurement point to the top of the atmosphere. Air is composed of molecules that have mass, and gravity pulls this mass toward the Earth’s surface. At sea level, the average column of air exerts a pressure of approximately 14.7 pounds per square inch.
This pressure is not constant and changes due to variations in altitude and temperature. As elevation increases, the amount of air overhead decreases, causing atmospheric pressure to drop. The standard atmospheric pressure at mean sea level is 1013.25 millibars (mbar), which is equivalent to 1013.25 hectopascals (hPa).
Pressure measurements are commonly expressed in these international meteorological units. Historical and common barometers often use inches of mercury (inHg) or millimeters of mercury (mmHg). The standard sea-level pressure is also equal to about 29.92 inches of mercury.
How the Mercury Barometer Works
The original mercury barometer, invented by Evangelista Torricelli in 1643, balances the weight of a mercury column against the external atmospheric pressure. The device consists of a glass tube, sealed at one end, filled with mercury and inverted into an open reservoir. The mercury falls until the column’s weight is counterbalanced by the force of the air pressing down on the reservoir.
The space created above the mercury column is a near-perfect vacuum, known as a Torricellian vacuum. When the atmospheric pressure increases, it pushes harder on the reservoir, forcing the mercury column higher up the tube. Conversely, a decrease in atmospheric pressure allows the mercury column to drop.
The height of the mercury column, typically around 30 inches at sea level, directly indicates the barometric pressure. The use of mercury is necessary because its high density allows the instrument to be a manageable size; a water-based barometer would require a tube over 34 feet tall.
How the Aneroid Barometer Works
The aneroid barometer, meaning “without fluid,” is the modern and most common type, relying on a mechanical system instead of liquid. Its mechanism centers on a small, sealed, flexible metal box called an aneroid cell or capsule. This cell, often made from an alloy like beryllium and copper, has most of the air pumped out, creating a partial vacuum inside.
The thin, corrugated walls of the capsule are highly sensitive to changes in the external atmospheric pressure. When the outside pressure rises, the cell is compressed inward, and when the pressure falls, the cell expands slightly. A system of mechanical linkages, including levers and springs, is attached to the flexible face of the capsule.
These levers translate the tiny inward or outward movements of the cell into a much larger, visible rotation. This amplified movement drives a pointer across a calibrated dial, which displays the pressure reading in units like millibars or inches of mercury. The precision of the aneroid barometer depends on the calibration of this mechanical linkage and the elasticity of the cell.
Interpreting Barometric Readings
Interpreting barometric readings involves observing both the current pressure value and the direction and speed of its change. Rising barometric pressure indicates the approach of fair, stable weather, often associated with a high-pressure system. A sustained period of high pressure, typically above 1013 hPa or 30.00 inHg, suggests clear skies and calm conditions.
Conversely, a falling barometric pressure signals that deteriorating weather is likely, as it indicates a low-pressure system is moving into the area. A slow, gradual drop may precede light rain or overcast conditions. A rapid and significant drop in pressure often forecasts a more severe event, such as a strong storm or high winds.
Barometers are also adapted for use as altimeters, particularly in aviation. This application uses the principle that air pressure decreases at a consistent rate with increasing altitude. By measuring the drop in pressure relative to a sea-level standard, the instrument can be calibrated to display an approximation of the height above the ground.