The primary use of a barometer is to measure atmospheric pressure. Barometers are important instruments in meteorology, enabling forecasters to predict weather changes. There are two main types of barometers: mercury and aneroid. Mercury barometers use liquid mercury in a glass tube, whereas aneroid barometers use a flexible metal chamber that expands or contracts with pressure changes. A rising barometer usually indicates fair weather, while a falling barometer can suggest that a storm is approaching. Barometers are essential tools for sailors, pilots, and meteorologists in understanding and anticipating weather patterns.