This is thanks to the way barometric pressure was first recorded. In 1643, a student of Galileo named Evangelista Torricelli invented the Mercury Barometer, which consists of a closed tube turned upside-down into a dish filled with mercury. As the weight of the air around the dish pushes down on the exposed mercury, some of the substance is forced up into the tube. Inch marks drawn on the outside of the tube are used to measure how far the mercury rises. On average, at sea level, the mercury rises 29.92 inches, as shown on the side of the barometer. In this case, then, the barometric pressure is reported as 29.92 inches of mercury.
---D.M.
