BAROMETERS, BAROGRAPHS, and ALTITUDE

Barometers measure changes in pressure. There are two ways pressure can change: weather systems ("highs" and "lows") and altitude.

At sea level, the average barometric pressure is approximately 29.92 inches of mercury, or 1013 millibars. At higher altitudes, there is less atmosphere weighing on us, and barometers read lower pressure. A rough rule of thumb is that for every 100' of increased altitude, the barometer will read about 0.10 inches lower, and for 1000', about 1.00 inches lower.

Example: if the sea level pressure is 30.00 inches, at 1000' feet it would be about 29.00 inches. At 5000', the pressure falls to about 25.00 inches.

If you are at an altitude of no more than a few thousand feet, and your barometer reads very low, then listen to a weather report (from a nearby station) for the barometric pressure, and set your barometer to agree with that. An adjustment screw on the back of your barometer can be turned with a small screwdriver. You will have corrected your barometer for altitude, and it will read what it would read at sea level.

If you live at several thousand feet above sea level, there is a chance that your barometer cannot be adjusted enough to compensate for a big change of pressure. Barometers allow only so much room for this correction. If trying to raise the position of the pointer does not result in sufficient correction, then your barometer is not going to function properly at that altitude. Some barometers will allow corrections for altitudes up to 10,000', but others to only 3000'. If yours cannot be adjusted enough, the only course is to find another location for it, at a lower altitude.