A barometer is a scientific instrument used to measure atmospheric pressure. It helps predict weather changes by indicating whether the pressure is rising or falling. When the pressure rises, it often suggests fair weather, while falling pressure can indicate storms or bad weather approaching. There are two main types of barometers: mercury barometers and aneroid barometers. A mercury barometer uses liquid mercury in a glass tube, while an aneroid barometer uses a small, flexible metal box that expands or contracts with pressure changes. Both types are essential tools in meteorology for understanding and forecasting weather patterns.