A barometer is the device used to measure atmospheric pressure. It can be either a mercury barometer, which uses liquid mercury to gauge pressure changes, or an aneroid barometer, which uses a small metal box that expands and contracts with pressure variations. Barometers are essential in meteorology for weather prediction, as changes in atmospheric pressure are closely linked to weather conditions. They are calibrated to measure pressure in different units, such as millibars or inches of mercury, and can provide insight into approaching weather systems, helping both amateurs and professionals in understanding weather patterns.