An instrument that measures air pressure is called a barometer. Barometers are essential tools in meteorology for predicting weather changes. There are two main types of barometers: mercury barometers and aneroid barometers. Mercury barometers use a column of mercury to measure atmospheric pressure, while aneroid barometers use a sealed metal chamber that expands or contracts with air pressure changes. As atmospheric pressure decreases, it often indicates that stormy weather is approaching, while an increase suggests fair weather. Understanding air pressure is crucial for navigation, weather forecasting, and understanding climate patterns.