Help calculating altitude from atmospheric pressure. Why my formula isn't outputting the correct result?

Question

The atmospheric pressure of my city is 1024 hPa. I want to turn that into altitude relative to sea level. I'm using the following formula to do so:

altitude = 44330 * [1 - (P/p0)^(1/5.255) ]

with p0=1013.25hPa (atmospheric pressure at sea level)

The problem is that the formula outputs -75m, which I know is not true for where I live. It should be anywhere between 30m and 40m above sea level. What am I doing wrong here? Should p0 be something else or am I using the wrong formula to get what I need?

Answer 1

You should not compare the current atmospheric pressure with the mean one at sea level.

The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

Local weather has a significant effect; high- and low pressure systems can easily increase or decrease the pressure by 40 hPa (at least here in Western Europe). You should compare your 1024 hPa with the current pressure at sea level, which is often not readily available. Instruments which use a barometer to measure altitude always have to be calibrated to account for weather differences.

To verify this yourself, check the barometer again in a few days, when it's exceptionally clear and stable weather (the pressure should be higher then) or when it's cloudy and stormy (the pressure should be much lower then). All this happens while your altitude doesn't change...