The instrument used to measure air pressure is a barometer. Barometers come in various forms, with the most common types being mercury barometers and aneroid barometers. Mercury barometers measure air pressure by the height of mercury in a glass tube, while aneroid barometers utilize a sealed metal chamber that expands or contracts with changes in air pressure. Barometers are essential tools in meteorology for forecasting weather changes, as fluctuations in air pressure can indicate approaching storms or fair weather conditions. They have also been instrumental in altimetry, helping to determine altitude based on pressure changes.