A barometer is the scientific instrument used by meteorologists to measure atmospheric pressure. There are three main types of barometers: mercury, aneroid and digital barometers.
A basic mercury barometer consists of a single open-ended glass tube and a reservoir. The glass tube is filled with mercury and is placed upside down in the reservoir, which also has mercury in it. The mercury level in the tube drops, creating a vacuum at the top. The barometer functions by balancing the weight exerted by the atmospheric pressure against the weight of the mercury in the glass tube. An aneroid barometer typically has a sealed metal chamber that contracts and expands based on the surrounding atmospheric pressure. A mechanical tool is used to measure how much the chamber contracts or expands. Digital barometers use electrical charges to accurately measure and display complicated atmospheric data.
A change in atmospheric pressure directly affects the weather. Meteorologists use barometers to forecast short-term weather changes. Atmospheric pressure that rises slowly, over a week, is usually an indication of settled weather that will likely last for a long time. An unexpected drop in atmospheric pressure within a few hours typically indicates an approaching storm, which will likely last a short period of time, accompanied by strong winds and heavy rain.