The instrument that measures atmospheric pressure is called a barometer. Barometers are essential tools in meteorology, helping to predict weather changes by measuring the weight of the atmosphere above a certain point. There are two main types of barometers: mercury barometers, which use mercury in a glass tube to measure pressure, and aneroid barometers, which use a sealed, flexible metal container that expands or contracts according to changes in pressure. Barometric readings are crucial for understanding weather patterns, particularly in forecasting storms and other weather events.