Why do cities at high altitude regions, such as Lhasa, altitude 11995ft (3656 metre), have high atmospheric pressures such as 30.11inHg (101.97 kPa) in the weather report?
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2$\begingroup$ Can you show an example of such a weather report? $\endgroup$– GimelistCommented Jun 5, 2017 at 8:46
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1$\begingroup$ 30.11 what? mm Hg, ft of water, grams per square thumb nail? What are your units? $\endgroup$– FredCommented Jun 5, 2017 at 12:52
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$\begingroup$ @Michael just google 'Lhasa atmospheric pressure', you'll see it, friend $\endgroup$– user8300Commented Jun 5, 2017 at 14:21
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1 Answer
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The reason for this is that the reported value is the relative pressure and not the absolute pressure. If meteorological stations reported the absolute pressure, dependent on elevation, it would been really confusing. So air pressure is always adjusted to sea level.
This is also why your home barometer has to be adjusted to show the correct pressure depending on the altitude where you are.
Here are a link to the weather close to where I live. It is at 150 m elevation, https://www.yr.no/place/Norway/Buskerud/R%c3%b8yken/B%c3%b8dalen/hour_by_hour_detailed.html?spr=eng If you seek for Drammen you will see the green line showing the same pressure but the elevation there is close to sea level 10m or so.
answered Jun 5, 2017 at 12:24
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2$\begingroup$ Thanks for your answer! But I don't quite understand why reporting the real pressure confusing. And if I want to see the real pressure where should I look at? And one more question, if I want to calculate the air density of a certain place, which air pressure should I use, the real pressure or the relative pressure? Thanks again! $\endgroup$– user8300Commented Jun 5, 2017 at 14:29
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3$\begingroup$ if you want to calculate air density then you use the real air pressure or as the correct name is the absolute pressure.i did google lhasa atmospheric pressure just now and the air pressure there was 1012 mb.the reason meteorologists use relative pressure is to make it simpler to collect and prosess data from a wide range of weather stations,if all stations barometers are calibrated to sea level it will be less numbers to handle. $\endgroup$ Commented Jun 5, 2017 at 15:01
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1$\begingroup$ if the elevation is known there is some fine pressure calculators on the net where you input the relative air pressure and your elevation to get the absolute pressure. $\endgroup$ Commented Jun 5, 2017 at 15:17
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$\begingroup$ I just found it. Solved my problem. Thanks for your help! $\endgroup$– user8300Commented Jun 5, 2017 at 15:53
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2$\begingroup$ keisan.casio.com/exec/system/1224575267 is one such calculation, just in case others are looking. Rule of thumb: 1 inch of pressure loss for every 1000 feet of elevation. $\endgroup$– user967Commented Jun 5, 2017 at 16:10