Barometers measure barometric pressure. Also known as atmospheric pressure, barometric pressure is defined as the "downward force exerted by the air above" and is measured in inches of mercury or in millibars. Barometers must be adjusted for the local altitude, or the height that the land is above sea level, because barometric pressure changes with altitude; the weather service and other meteorological organisations use sea-level barometric pressure as the standard. To keep your readings in tune with the professionals so you can track the weather accurately, you must adjust your barometer.
Determine your elevation in feet (or meters) above sea level. Most good GPS units will tell you, or you can take your local elevation from a topographic map.
Use the sea-level conversion chart provided with your barometer, if your barometer has one. Typically, there will be an elevation level at the top of a bar, with the correction from sea level directly underneath. Determine what your local elevation is in feet (or meters, if need be) and then look directly underneath for your correction.
Turn the adjusting screw on the back of your barometer to match the correction that you just determined. Refer to your instruction manual if your barometer does not have an adjustment screw and follow the directions on how to adjust for altitude.
Use a conversion table to manually correct the barometer for elevation. There are many conversion tables available on the Internet (see Resources).
Add the appropriate amount of millibars for your elevation level, if your barometer measures in millibars. For example, add 18.2 millibars if you are 500 feet above sea level, 36.1 millibars if you are 1,000 feet above sea level.
Add the appropriate amount of inches of mercury for your elevation level, if your barometer measures in inches of mercury. For example, add 0.54 inches of mercury if you are 500 feet above sea level, 1.07 inches of mercury if you are 1,000 feet above sea level.
You can also adjust your barometer by matching it to the barometric pressure as detailed by your local weather service office. The readings provided by the weather service and local television stations and other news organisations will already be adjusted for standard, sea-level barometric pressure. Adjust your barometer for altitude when you first purchase the unit and then again once a year.