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List:       flightgear-devel
Subject:    Re: [Flightgear-devel] Weird Sun and weather bugs
From:       Ron Jensen <wino () jentronics ! com>
Date:       2009-09-23 4:41:41
Message-ID: 1253680901.13499.17.camel () ip120 ! jentronics ! com
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I still see a problem with altimeters and pressures.  

Sitting at KHIF (4789 ft) with a metar that reads:
KHIF 230355Z 09006KT 10SM CLR 14/04 A3032 RMK AO2A DZE0300 SLP227 P0000 T01380036 $

I set my altimeter to 30.32.  The altimeter reads 4666 ft, or about 120
feet too low.

/environment/pressure-inhg reads 25.56.

Using the METAR formula:

 Q = (Pa^0.1903 + 1.313e-5*Ha)^(1/0.1903)

Where Q is the altimeter setting, Pa is airfield pressure inHg, and Ha
is airfield height in feet.  I get an altimeter setting of 30.46, which
makes my altimeter read 4793 ft.

Working that formula backwards using the METAR altimeter setting to get
Pa:

 Pa = (Q^0.1903 - 1.313e-5*Ha)^(1/0.1903)

Yields a field pressure of 25.44 inhg.


In a nutshell, it looks like we are using the METAR altimeter setting
for /environment/pressure-sea-level-inhg instead of calculating the
correct sea-level pressure to give a field pressure based on a
non-standard lapse rate.


Thanks,

Ron


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