High Flying Effects
Barometers are calibrated to sea level pressure. However, as altitude increases, the rate-of-change of pressure with altitude does not make a linear match. In other words:
The bad news is mercury-filled barometers cannot be effectively calibrated at altitude. Besides, these barometers must also make a complicated gravity correction. Aneroid barometers, however, have a scale that can be adjusted as needed. They are usually accurate enough for casual barometer use. By adjusting this scale, the barometer reading is offset for the correction.
An Important Note
Though adjusted, the barometer cannot certainly display an accurate reading as dynamic pressure systems come in and out of the area. That said, the best way to ensure barometers are properly compensating for altitude difference is through a comparison. Agencies such as the National Oceanic and Atmospheric Administration (NOAA) continually broadcast weather information for hundreds of regions. Check in with their barometer readings to keep yours timely and accurate.