Measuring altitude with barometric pressure is not free from difficulties, however. Atmospheric pressure can also change with changes in weather patterns. On an uneventful weather day it is not too unusual for an air pressure change of 1 mbar, caused by temperature changes alone. This 1 mbar change in pressure could result in a skewed altitude reading of up to 26 feet (8 meters). But on an afternoon full of weather changes, like an approaching cold front, air pressure could change by as much as 5 mbar, or more. This could result in a skewed altitude reading of up to 130 feet (40 meters), or more.
Typically, when bad weather is approaching, the pressure will be falling. And the altimeter thinks this decrease in pressure is due to an increase in altitude, so it will read higher than you really are. And the opposite is the case when the weather conditions are improving – the altimeter will read lower than you really are.
In order to compensate for weather changes, an altimeter must be calibrated using a known altitude or a known pressure value. A known altitude can typically be taken from a specific landmark on a topographic map. But if you do not have a known altitude to use, you will need to use a known pressure value. This is typically done with a barometric pressure value. "Barometric Pressure" is the current air pressure at sea level, for a specific location. Barometric pressure is measured several times per day, and can usually be obtained from flight service stations or aviation weather reports.
With all of the changes that affect an altimeter, your elevation can easily be skewed by a few hundred feet in as little as one day’s time. You will want to calibrate your unit at least once per day, but for improved accuracy (especially in changing weather conditions), you may want to calibrate it every couple of hours at a known elevation.
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