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In various atmospheric equations, surface pressure is one of the variables used. For example, in calculating the density of air or pressure-corrected relative optical air mass. How is surface pressure measured?

I imagine it's the pressure at the height of a sensor on a weather station. Is that considered effectively pressure at Earth's surface / a good-enough approximation, or do scientists generally adjust measurements from weather stations to be corrected for the height difference between the sensor and Earth's surface? Or is surface pressure measured by a sensor actually at Earth's surface?

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When you look on meteorological maps the "surface pressure" is actually the reduced MSLP (mean sea-level pressure) value. See this NWS page for more info. So whatever height the sensor is at above the ground, they regardless compare that height back versus the sea-level to report the pressure.

I always figured barometers were sited along with the main instrument set. Those indeed will all generally be raised up off the ground certain heights (aside from any soil temperature thermometers) to prevent issues with any standing water or snow cover. See for example the instrument list and extensive pictures for Oklahoma Mesonet site locations (slightly different setup, but the 2 meter temperature and 10 meter wind are widespread standards).

But I was surprised to read in the ASOS User's Guide that:

At virtually all locations, the pressure sensors are located indoors within the ACU.

And:

The ACU, which is the central processing unit for the ASOS, is usually located inside a climate controlled structure, such as an observing office or control tower building. It ingests data from the DCP(s) and pressure sensors, and is capable of accepting information from the FAA New Generation Runway Visual Range (NGRVR) system.

So it sounds like the barometer is often not on site, and there are indeed no specifications for height. But since it's reduced to sea level regardless, the elevation is all that really matters to reduce it to MSLP, and so some variation in height of the sensor shouldn't matter.

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"The instrument for measuring atmospheric pressure is the barometer. Meteorologists apply standards defined by the World Meteorological Organization to compare pressure measurements among themselves. They are thus brought back to sea level and the barometers are calibrated to indicate the pressure at sea level. This correction amounts to adding the weight of the missing air column between the altitude of the measuring point and the sea level (or a rare case, to subtract it for a measurement point located under the surface of the sea)."

source: meteofrance.

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