A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. A simple barometer 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—that is, as the air becomes heavier—it pushes more of the mercury up into the tube. As air pressure decreases, more of the mercury drains from the tube. So the level of mercury in the tube provides a precise measure of air pressure.