How is air pressure measured for weather information?
A barometer is an instrument that is used for measuring air pressure. In it's simplest form it is a cup of mercury with a vacuum tube in it. The weight of the atmosphere on the surface of the mercury will drive the mercury up into the tube. Earth's standard atmospheric pressure is 29.92 inches of mercury. In other words the average weight of the atmosphere would push the mercury in the cup 29.92 inches up the vacuum tube.