What is a Barometer

A barometer is a scientific instrument used to measure atmospheric pressure, which is the force exerted by the weight of the air above the Earth's surface. Think of the atmosphere as a vast ocean of air. Just as the pressure increases the deeper you dive into water, atmospheric pressure changes depending on your altitude and the weather. This pressure is an important factor for meteorologists and scientists to predict weather patterns. Changes in barometric pressure can signal an approaching weather system, making the barometer a valuable tool.

There are two primary types of barometers: mercury and aneroid. A mercury barometer is a simple yet accurate device. It consists of a glass tube with a sealed top, placed upside down in a small pool of mercury. The atmospheric pressure pushes down on the mercury in the pool, forcing some of the liquid metal up into the tube. The height of the mercury column in the tube is a direct measurement of the air pressure. A high column indicates high pressure, and a low column indicates low pressure.

The second type, an aneroid barometer, is more common today because it is smaller, safer, and does not use toxic mercury. The word "aneroid" means "without fluid." Inside this type of barometer is a small, sealed, flexible metal box called an aneroid capsule. When the atmospheric pressure increases, it squeezes the capsule, causing it to compress slightly. When the pressure decreases, the capsule expands. This tiny movement is amplified by a system of levers and springs, which moves a needle on a dial, providing a pressure reading.

Barometers have many real-world applications beyond simple weather forecasting. Pilots use them as altimeters—instruments that measure altitude. As a plane climbs higher, the air pressure decreases, and the altimeter translates this pressure drop into a measurement of height above sea level. Many smartphones and fitness trackers also contain tiny digital barometers to measure altitude changes for activities like hiking or climbing stairs. By monitoring the changes in air pressure, barometers give us valuable clues about our environment.

Fun Fact: The first barometer was invented in 1643 by Italian physicist Evangelista Torricelli, a student of Galileo.