An instrument often used to measure atmospheric pressure is a barometer. Barometers work based on the principle that air pressure changes with weather conditions, affecting the height of a column of liquid within the device—most commonly mercury or water. There are two main types of barometers: mercury barometers, which utilize mercury in a glass tube, and aneroid barometers, which use a flexible metal chamber that expands or contracts with pressure changes. Barometers are essential tools in meteorology, helping predict weather patterns by indicating whether the atmospheric pressure is rising or falling.