A barometer is a scientific instrument used to measure atmospheric pressure, also called barometric pressure. The most common type of barometer is a mercury barometer, which consists of a long glass tube (closed at one end, open at the other) filled with mercury and turned upside down into a container of mercury. The barometer works by balancing the mercury in the glass tube against the outside air pressure, just like a set of scales. As air pressure increases, it pushes more of the mercury up into the tube, and as air pressure decreases, more of the mercury drains from the tube.

There are two types of barometers: a mercury barometer, measuring pressure in inches of mercury, and an aneroid barometer that uses sealed wafers that shrink or expand in different pressures. Atmospheric pressure is also known as barometric pressure because it is measured using a barometer. The standard unit for measuring barometric pressure is called an atmosphere (atm). In relation to the English system of measurement, one atmosphere (atm) equals 29.9213 inches of mercury (inHg). In both millibars (mb) and hectopascal (hPa), the standard pressure at sea level is 1013.25 mb or hPa.

In summary, a barometer is used to measure atmospheric pressure, and the most common type is a mercury barometer. The standard unit for measuring barometric pressure is called an atmosphere (atm), and it can also be measured in millibars (mb) and hectopascal (hPa) .