The instrument used to measure air pressure is called a barometer. Barometers are essential tools in meteorology for understanding and predicting weather conditions. There are two main types of barometers: mercury and aneroid. Mercury barometers utilize liquid mercury in a glass tube, while aneroid barometers use a metal capsule that expands and contracts in response to changes in air pressure. As atmospheric pressure decreases, it often indicates approaching storms, while an increase may signify fair weather. Barometers are widely used in aviation and meteorology to provide accurate data for weather forecasting and analysis.