What Does a Barometer Measure?

The barometer is one of the more poorly understood, but generally known, scientific instruments. It has been around for centuries, and virtually everyone has seen more than one movie or TV show where “the falling barometer” was mentioned in connection with a storm at sea or coming bad weather. Despite most everyone knowing what a barometer is for, few really know how one works.


  • A barometer is a device used to measure atmospheric pressure.


  • There are three major types of barometers: aneroid, mercury and water. An aneroid barometer uses a cell made of beryllium and copper. The two layers of metal are separated by springs, which expand and contract with changes in air pressure. Mercury barometers are the best-known, and what are typically imagined when people think of barometers. This is a tube with an open reservoir of mercury at the base, visually similar to a mercury thermometer. The mercury fills or empties the tube to balance the pressure placed on the reservoir. The water reservoir looks like a sealed, clear-glass pitcher with a spout. The spout is open to the atmosphere and connects to the body below water level, and increased pressure will push the water level in the spout below that of the body.


  • Changes in air pressure can be used to forecast the weather. In particular, dropping air pressure is associated with bad weather. When combined with wind data, the barometer can be a powerful tool for monitoring and making short-term predictions on the weather.


  • Improved communications technology (the telegraph) allowed barometric monitoring stations to be set up, allowing pressure to be checked simultaneously across a wide area. The result were the maps featuring weather “fronts” that are common features in newspaper and television weather forecasts today. These indicate the border between areas of high and low air pressure.


  • The barometer does have some problems. Mercury barometers and water barometers have problems with temperature. Mercury will expand and contract not just with the air pressure, but also with temperature. This is why they are often part of a larger instrument including a thermometer, so temperature can be factored into air pressure readings. Water barometers can freeze, making them of dubious value in northern climates during the winter season. Also, all barometers require adjustment based on altitude, as air density decreases at higher levels. Most barometric guides are set for pressure at sea level, so a reading from on top of a mountain must take the natural decrease in density and pressure into account.