A barometer measures atmospheric pressure. It is an essential tool in meteorology, used to forecast weather changes. When the atmospheric pressure changes, it can indicate whether weather conditions are improving or deteriorating. For example, rising pressure often signals clear, fair weather, while falling pressure suggests an approaching storm. There are two main types of barometers: mercury and aneroid. Each has its mechanism, but both serve the same fundamental purpose of gauging air pressure to aid in weather prediction.