Editing Barometer (section)

=== Altitude ===
[[File:NWS Key West office barometer as Hurricane Irma approached.jpg|thumb|A digital barometer with altimeter setting (for correction) displayed]]

As the air pressure decreases at [[altitude]]s above sea level (and increases below sea level) the uncorrected reading of the barometer will depend on its location. The reading is then adjusted to an equivalent sea-level pressure for purposes of reporting. For example, if a barometer located at sea level and under fair weather conditions is moved to an altitude of 1,000 feet (305&nbsp;m), about 1 inch of mercury (~35 hPa) must be added on to the reading. The barometer readings at the two locations should be the same if there are negligible changes in time, horizontal distance, and temperature. If this were not done, there would be a false indication of an approaching storm at the higher elevation.

Aneroid barometers have a mechanical adjustment that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude. Setting an aneroid barometer is similar to resetting an [[Clock face|analog clock]] that is not at the correct time. Its dial is rotated so that the current atmospheric pressure from a known accurate and nearby barometer (such as the local [[weather station]]) is displayed. No calculation is needed, as the source barometer reading has already been converted to equivalent sea-level pressure, and this is transferred to the barometer being set—regardless of its altitude. Though somewhat rare, a few aneroid barometers intended for monitoring the weather are calibrated to manually adjust for altitude. In this case, knowing ''either'' the altitude or the current atmospheric pressure would be sufficient for future accurate readings.

The table below shows examples for three locations in the city of [[San Francisco]], [[California]]. Note the corrected barometer readings are identical, and based on equivalent sea-level pressure. (Assume a temperature of 15&nbsp;°C.)

{| class="wikitable" style=text-align:right
|-
! Location !! Altitude<br>(feet) !! Uncorrected P<sub>atm</sub><br>(inches Hg) !! Corrected P<sub>atm</sub><br>(inches Hg)!! !! Altitude<br>(metres) !! Uncorrected P<sub>atm</sub><br>(hPa) !! Corrected P<sub>atm</sub><br>(hPa)
|-
| '''City Marina''' || (Sea Level) 0 ||29.92 || 29.92 ||  || 0  ||1013 hPa || 1013 hPa
|-
| '''Nob Hill''' || 348 || 29.55 || 29.92 || || 106  || 1001 hPa || 1013 hPa
|-
| '''Mt. Davidson''' || 928 || 28.94 || 29.92 || || 283  || 980 hPa || 1013 hPa
|}

In 1787, during a scientific expedition on [[Mont Blanc]], [[Horace Bénédict de Saussure|De Saussure]] undertook research and executed physical experiments on the [[Water#States|boiling point of water]] at different heights. He calculated the height at each of his experiments by measuring how long it took an alcohol burner to boil an amount of water, and by these means he determined the height of the mountain to be 4775 metres. (This later turned out to be 32 metres less than the actual height of 4807 metres). For these experiments De Saussure brought specific scientific equipment, such as a barometer and [[thermometer]]. His calculated boiling temperature of water at the top of the mountain was fairly accurate, only off by 0.1 kelvin.<ref>{{cite web|url=http://physicspure.blogspot.com/2020/02/what-is-kelvin-scale-of-temperature.html|title=Kelvin scale in depth|access-date=12 February 2020}}{{Dead link|date=July 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Based on his findings, the [[altimeter]] could be developed as a specific application of the barometer. In the mid-19th century, this method was used by explorers.<ref>{{cite journal |first1=M. N. |last1=Berberan-Santos |first2=E. N. |last2=Bodunov |first3=L. |last3=Pogliani |title=On the barometric formula |journal=American Journal of Physics |volume=65 |issue=5 |pages=404–412 |year=1997 |doi=10.1119/1.18555 |bibcode = 1997AmJPh..65..404B  }}</ref>