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A rapid drop in barometric pressure means disturbances nearby, showers of short duration.

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Regular elevation in barometric pressure usually will indicate a clear, dry weather conditions

(cold and dry in the winter).

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A slow but continuous drop in barometric pressure will indicate persistent, bad weather.

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Slow drops of 2-3 tenths mbar per 24 hours a depression of some distance away.

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Drops of 1-2 tenths mbar per hour means disturbances nearby of short duration.

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Steep drops of 6-10 tenths mbar within 4-5 hours period indicates coming rain and/or storm

with strong winds.

4.3 Absolute vs. Relative Pressure

To compare pressure conditions from one location to another, meteorologists correct pressure to

sea-level conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected

pressure (the pressure your location would be at if located at sea-level) is generally higher than your

measured pressure.

Thus, your absolute pressure (measured at your location) may read 28.62 inHg (969 mb) at an altitude

of 1000 feet (305 m), but the relative pressure (sea-level) is 30.00 inHg (1016 mb).

The standard sea-level pressure is 29.92 in Hg (1013 mb). This is the average sea-level pressure

around the world.

Relative pressure measurements greater than 29.92 inHg (1013 mb) are

considered high pressure and relative pressure measurements less than 29.92 inHg are considered low

pressure.

4.4 Barometer Calibration

Your barometer can not be pre-calibrated at the factory for your altitude, so you must calibrate the

barometer for your specific location.

To determine the relative pressure for your location, locate an official reporting station near you (the

internet is the best source for real time barometer conditions, such as Weather.com or

Wunderground.com), and set your weather station to match the official reporting station.

Your barometer will operate from -100 to 3,000 feet with reliable accuracy. Aneroid barometers have a

small screw on the back for sea-level calibration (reference Figure 1).

With a flat blade screwdriver, turn this screw in either direction slightly while looking at the indicator

needle. Turn clockwise (+) to increase the barometer and counter-clockwise (-) to decrease the

barometer.

Continue this procedure until the proper pressure reading is obtained. Do not turn the screw

counter-clockwise (to the left) too far, since the screw can fall out. If the barometer needle exceeds

the measurement range, do not continue to turn the set screw. This is likely occurring because you live

outside the barometer operating range.

After the initial calibration, no further adjustment will be required unless the barometer is moved to a

new geographic location.

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