A barometer measures atmospheric pressure. The air in the atmosphere exerts a force called pressure that constantly changes due to moving weather systems. Therefore, in conjunction with other meteorological instruments, a barometer helps to predict clear or rainy weather.

In 1643, Evangelista Torricelli invented the first barometer. He figured that if he had a vacuum, an airless space, he could compare the ever-present atmospheric pressure with zero pressure. So he placed a vacuum contained in a glass tube on top of a larger container of mercury. The air presses down on the mercury's exposed surface and pushes it up into the tube. The higher the mercury level, the greater the air pressure. Originally, the units of air pressure were just millimeters of mercury. People could finally measure the force of air.

Since the advent of "Torricelli's tube," others developed the aneroid barometer that works without liquid. In this instrument, a flexible metal accordion box that resembles a bellows is partially squeezed to a medium pressure. Then, if air pressure rises, the bellows contract because the air inside them takes up less volume. An aneroid barometer often connects to a recording device, together forming a barograph. A pen moves against a rotating cylinder whenever the bellows moves, and thus creates a visual aid to the pattern of falling and rising air pressure. The barograph remains a basic instrument of modern meteorology.

Used in conjunction with a thermometer, a barometer can make general weather predictions. While weather is very complex, storms more or less follow certain patterns of high and low pressure systems. In simplified terms, a rising barometer means wind, frost, or clear skies, while a falling barometer indicates coming storms. A steady barometer might mean precipitation or sun. Weather forecasters look at the relative change at different places, taking into account how air pressure changes with elevation.

Some home hobbyist barometers still convert units to inches or millimeters of mercury, whether or not mercury is present. The International Meteorological Society has declared the universal unit of pressure to be the hectopascal. For example, 1016 hectopascals equals 30 inches of mercury (762.1 mm).