Altimeter to Station Pressure

[jde...@profitmaker.com]

Can someone help me with the following problem? At our station, we seem
to have a problem with converting altimeter setting to station pressure.
The problem arises from differing interpolations from using the pressure
reduction computer. For instance, one person will reduce, say, 29.92 to
29.100 while another observer will reduce it to 29.095 or 29.105. Our
staion elevation is 769ft. and I would like to construct a table to
achieve some continuity in our pressure readings, especially for the
purpose of the app group on our three and six hourlies. Is there a
simple formula for constructing a table with known elevation and
altimeter settings? I'm sure the Smithsonian tables would have this
information, but alas, I don't have a copy.

[Tom Whittaker]

jde...@profitmaker.com writes:

>Can someone help me with the following problem? At our station, we seem
>to have a problem with converting altimeter setting to station pressure.

The altimeter setting is derived from the station pressure assuming
a Standard Atmosphere between the station elevation and sea level.

The formula: SP = ALT * ( ( (288 - (Z * 6.5E-3) )/288) ** 5.256)

where: Z = station elevation in meters
ALT = altimeter setting
SP = station pressure

Will do the conversion...for your example: at 769 feet (234.39
meters), an altimeter 29.92 equals a station pressure of 29.097
(or 29.10 with rounding)...

--
Tom Whittaker to...@ssec.wisc.edu
Space Science and Eng. Center University of Wisconsin-Madison

[Keith Brewster]

In article <dlr0273.3...@silver.sdsmt.edu> dlr...@silver.sdsmt.edu (DARAN L. RIFE) writes:
>In article <20908KNSJ...@profitmaker.com> jde...@profitmaker.com writes:
>>Date: Sat, 17 Sep 94 16:29:32 EST
>>From: jde...@profitmaker.com
>>Subject: Altimeter to Station Pressure

>The problem with what you are asking is that a "standard table" would be
>impossible. Reduction of station pressure to sea level pressure normally
>requires using the virtual temperature averaged over the last 12 hours at the
>station.

This statement is true for computing sea level pressure from station
pressure, but _there is_ a standard table for computing altimeter
setting from station pressure, and vice versa. This is because
the altimeter is programmed for a specific temperature profile.
A quick answer is to provide a subroutine I use:

FUNCTION alttostpr(altim,elev)
implicit none
C
C Arguments
C
real alttostpr ! station pressure
real altim ! altimeter setting
real elev ! elevation in meters above sea level
C
C Physical constants
C
REAL To,gamusd,rdgas,g,c1
PARAMETER ( To = 288., ! degrees K sea-level temp in US std atmos
+ gamusd = 0.0065, ! K /m std atmos lapse rate
+ rdgas = 287.04, ! J/(K*kg) gas constant for dry air
+ g = 9.80616, ! m/(s*s)
+ c1 = (g/(gamusd*rdgas)) )
C
c print *, ' Altimeter setting = ',altim
alttostpr=altim*(1.-(elev*gamusd/To)) ** c1
c print *, ' station pressure',alttostpr
RETURN
END

I believe the correct elevation to provide is cockpit height, which
is surface elevation plus 3 meters. Pressures may be in inches of Hg
or mb (or Pa for that matter).

Keith

-----------------------------
Keith Brewster
Center for Analysis and Prediction of Storms
OU School O'Meteorology
cap...@alliant.backbone.uoknor.edu

[Tom Whittaker]

dlr...@silver.sdsmt.edu (DARAN L. RIFE) writes:

>The problem with what you are asking is that a "standard table" would be
>impossible. Reduction of station pressure to sea level pressure normally
>requires using the virtual temperature averaged over the last 12 hours at the

>station. There is a relatively simple equation one can use to determine the
>station pressure for a given value of sea level pressure (altimeter) and
>vice-versa. This equation has the form (please excuse the fact that this
>server does not have an equation writer):

It is very important here to make the distinction between "altimeter
setting" and "sea level pressure".

As Daran points out, the "sea level pressure" is based on formulae that
use time-averaged temperature, plus, in some cases, other station-
dependent fudges, like the infamous "plateau correction".

The altimeter setting, however, is not sensitive to the observed
temperature, but has, as I indicated in my previous post, a built-in
assumption of the ICAN Standard Atmosphere (which includes such things
as a 'standard sea level pressure' of 1013.25mb, a 'standard lapse
rate' below 11km of .0065 C/m, a 'standard sea-level temperature' of
288 K).

Because of this, it is practical to make a table for converting
'altimeter setting' to 'station pressure', but not for converting 'sea
level pressure' to 'station pressure'.

Hope that helps...