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From: wolit@alice.UUCP (Jan Wolitzky)
Newsgroups: net.aviation
Subject: Re: pressure vs altitude
Message-ID: <3481@alice.UUCP>
Date: Thu, 21-Mar-85 09:20:37 EST
Article-I.D.: alice.3481
Posted: Thu Mar 21 09:20:37 1985
Date-Received: Fri, 22-Mar-85 02:24:16 EST
Organization: Bell Labs, Murray Hill
Lines: 8

As I recall, for each 18,000 (approx) of altitude, the pressure drops 
by half.  Thus, the pressure at an altitude of X feet, in atmospheres,
is something like: 2 ^ -(X / 18000).
Multiply by 14.7 for PSI, 29.98 for inches of Hg, etc.
Of course, temperature, humidity, latitude, and plain old deviation 
from ideal gas behavior all have their effect.
-- 
Jan Wolitzky, AT&T Bell Laboratories, Murray Hill, NJ; (201) 582-2998