An instrument commonly used to measure atmospheric pressure is the barometer. Barometers can be classified into two main types: mercury and aneroid. The mercury barometer uses a column of mercury in a glass tube to indicate pressure changes, while the aneroid barometer relies on a small, flexible metal box that expands or contracts with pressure variations. Barometers are essential for weather forecasting, as changes in atmospheric pressure can indicate shifts in weather patterns. By monitoring pressure changes, meteorologists can predict storms or clear skies, making this instrument crucial in environmental science.