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altimeter to slp (fwd)


===============================================================================
Robb Kambic                                Unidata Program Center
Software Engineer III                      Univ. Corp for Atmospheric Research
address@hidden             WWW: http://www.unidata.ucar.edu/
===============================================================================

---------- Forwarded message ----------
Date: 24 Mar 2005 15:16:49 -0700
From: Steve Chiswell <address@hidden>
To: Robb Kambic <address@hidden>
Subject: altimeter to slp

Robb,

In PR_PALT,

the constants are:
GAMUSD = 6.5 degrees per km
TMCK = 273.15
To = T at 1000mb = 288.15K
RDGAS = 287.04 = gas constant for dry air
GRAVTY = 9.80616 m s-2

Chiz
        FUNCTION PR_PALT  ( altm, selv )
C************************************************************************
C* PR_PALT                                                              *
C*                                                                      *
C* This function computes station pressure from altimeter and station   *
C* elevation.  The following equation is used:                          *
C*                                                                      *
C*     PALT = ALTM * ( 1 - ( SELK * GAMUSD / To ) ) ** expo             *
C*                                                                      *
C*          SELK  =  SELV / 1000                                        *
C*            To  =  US Std. Atmos. sea level temp in Kelvin            *
C*                =  TMCK + 15                                          *
C*          expo  =  GRAVTY / ( GAMUSD * RDGAS ) * 1000                 *
C*                                                                      *
C* Wallace and Hobbs.                                                   *
C*                                                                      *
C* REAL PR_PALT  ( ALTM, SELV )                                         *
C*                                                                      *
C* Input parameters:                                                    *
C*      ALTM            REAL            Altimeter in millibars          *
C*      SELV            REAL            Station elevation in meters     *
C*                                                                      *
C* Output parameters:                                                   *
C*      PR_PALT         REAL            Pressure in millibars           *
C**                                                                     *
C* Log:                                                                 *
C* I. Graffman/RDS      11/84   Original source                         *
C* I. Graffman/RDS      12/87   Use constants from table                *
C* G. Huffman/GSC        7/88   Documentation                           *
C* T. Lee/GSC           12/99   Used TMCK                               *
C************************************************************************
        INCLUDE         'GEMPRM.PRM'
        INCLUDE         'ERMISS.FNC'
C------------------------------------------------------------------------
C*      Check for missing data.
C
        to = TMCK + 15.
        IF  ( ERMISS ( altm ) .or. ERMISS ( selv ) )  THEN
            PR_PALT = RMISSD
          ELSE
            hgtk = PR_HGMK (selv)
C
C*          Calculate the exponent.
C
            expo = ( GRAVTY / ( GAMUSD * RDGAS ) * 1000.)
C
C*          Calculate pressure.
C
            PR_PALT = altm * ( 1. - ( hgtk * GAMUSD / to ) ) ** expo
        END IF
C*
        RETURN
        END