Erica's Answer:
The unit "inch of mercury" is determined by the atmospheric pressure. Pressure is literally the weight of the atmosphere at a location. A barometer uses a principle that was discovered centuries ago: If a vacuum tube (containing no air) is placed upside down in a pool of mercury, the mercury will rise or fall within the tube to counterbalance the weight of the atmosphere outside of the tube. If the mercury in the vacuum tube rises, that means the atmospheric pressure is rising, which is a sign of improving weather conditions. However, if the mercury falls, the pressure is getting lower, which means clouds (and perhaps even a storm) are heading our way.- Erica
