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 is high, it usually means clear skies, while low pressure often signals storms or rain. There are two main types of barometers: the mercury barometer and the aneroid barometer. The mercury barometer uses liquid mercury in a glass tube, while the aneroid barometer uses a small, flexible metal box. Both types are essential tools for meteorologists and anyone interested in understanding the weather, including sailors and hikers.