A barometer is the instrument used to measure atmospheric pressure. It operates based on principles of physics, typically employing either a mercury column or an aneroid system. The mercury barometer uses a tube filled with mercury inverted in a basin, where changes in atmospheric pressure cause the mercury level to rise or fall. Aneroid barometers contain a sealed metallic box which expands or contracts as atmospheric pressure changes. Both types provide vital information for weather prediction and understanding atmospheric phenomena, making them essential tools in meteorology and environmental science.