A barometer is an instrument that measures atmospheric pressure, the force exerted by the air surrounding Earth. This pressure constantly changes due to environmental factors.
The Concept of Atmospheric Pressure
Atmospheric pressure represents the weight of the air column above a given point on Earth’s surface. Air molecules are pulled towards Earth by gravity, resulting in this measurable force. At sea level, there is a substantial column of air above, leading to higher pressure. As altitude increases, the amount of air above decreases, causing atmospheric pressure to decline.
Temperature also influences atmospheric pressure. Warm air molecules move faster and spread out, making the air less dense and resulting in lower pressure. Conversely, colder air is denser, with molecules packed more closely, which leads to higher pressure.
Common units for measuring atmospheric pressure include hectopascals (hPa), millibars (mb), and inches of mercury (inHg). One standard atmosphere at sea level is approximately 1013.25 hPa or 29.92 inches of mercury.
How Barometers Work
Evangelista Torricelli invented the first mercury barometer in 1643. These barometers consist of a vertical glass tube, sealed at the top and open at the bottom, inverted into a reservoir of mercury. Atmospheric pressure pushes down on the mercury in the reservoir, forcing it up the tube until the weight of the mercury column balances the external air pressure. The height of the mercury in the tube indicates the atmospheric pressure.
Aneroid barometers (“without liquid”) offer a more portable and safer alternative. They contain a small, flexible, sealed metal capsule with most of the air removed. As external atmospheric pressure changes, this “aneroid cell” expands or contracts. A system of mechanical levers and springs magnifies these tiny movements, translating them into the rotation of a pointer across a calibrated dial, indicating the pressure.
Modern digital barometers utilize advanced pressure sensors, often based on Micro-Electro-Mechanical Systems (MEMS) technology. These sensors convert pressure changes into electrical signals. Some common types are capacitive or piezoresistive sensors, where pressure deforms a diaphragm, altering electrical properties that are then measured and displayed digitally.
Everyday Uses of Barometers
Barometers have practical applications, notably in weather forecasting. Meteorologists monitor changes in atmospheric pressure to predict upcoming weather patterns. A rapid drop in pressure often signals approaching stormy conditions, such as rain or snow, due to low-pressure systems. Conversely, rising or stable high pressure indicates clear, fair weather.
Barometers are also used for measuring altitude, particularly in instruments known as altimeters. Since atmospheric pressure decreases predictably with increasing height, these devices can determine elevation based on pressure readings. Altimeters are widely used in aviation to help pilots maintain flight levels and are common among hikers and mountaineers for navigation and safety. Beyond these traditional uses, pressure sensors, essentially miniature barometers, are integrated into many modern devices, including smartphones and drones, providing environmental data.