You hear it every day during our weather forecasts–the barometer or barometric pressure is rising or falling. A barometer is a weather instrument that is used to measure the air pressure of the atmosphere. By observing the pressure tendency (whether it’s rising or falling), meteorologists can forecast short term changes in the weather and by collecting multiple barometric readings over a wide area, meteorologists can make a surface weather analysis areas of low and high pressure, troughs and warm and cold fronts.
The first barometers were fashioned from long vacuum filled tubes that connected with an open reservoir of Mercury (Hg). As the air pressure got higher, more pressure would be exerted on the reservoir of Mercury pushing it upward in the tube. Conversely, as the atmospheric pressure got lower, there would be less pressure on the reservoir and the level of Mercury in the tube would fall. Hence the terms rising and falling when referring to barometric pressure.
While Mercury barometers are the most accurate way of measure atmospheric pressure, most people are more familiar with aneroid barometers. These type of barometers contain a small, flexible metal box made from an alloy of Beryllium (Be) and Copper (Cu). This vacuum sealed box is mechanically connected to a needle and dial so as the atmospheric pressure goes up or down, the needle displays the movement and measurement.
In the United States and a few other countries, the unit of measurement for barometric pressure is in inches or millimeters (mm) where the standard or average pressure is 29.92″ or 760mm. Most barometers have a display that not only shows the measurement by also the type of weather to be expected with a specific reading (Stormy, Rain, Change, Fair and Very Dry). Other countries measure pressure in hectoPascals (hPa) or millibars (mb).