20030801: pressures

Jimmy,

ALTM is the altimeter setting in millibars.
PMSL is the reduced sea level pressure. 
Plotting either of these parameters gives only the values reported
for that parameter in the surface file. No other conversion is done.

I provided a convenience surface parameter "PANY" (aka pressure any) 
to display the reduced sea level pressure PMSL if available, otherwise 
use the value from ALTM if available.

PALT is the station pressure calculated from the altimeter setting 
according to a standard atmosphere equation/lapse rate. This
is not a sea level pressure.

The ALTM and PMSL values are not strictly equal since the altimeter setting 
reduction typically uses a time averaged virtual temperature for the column
of air that would be below ground. The comparison between the 2 will
not be as good for higher elevation stations.

You should consult the ferdral meteorological handbook for
observing practices if you are unsure about the relationship
between reported altimeter and reduced sea level pressure.

Steve Chiswell




>From: address@hidden
>Organization: UCAR/Unidata
>Keywords: 200308011519.h71FJRLd015895

>Hi-
>I have altimeter setting from surface stations. after poking around in the 
>gempak library, I realized that converting from ALTI to ALTM is a simple 
>multiplication conversion. I understand that all metar stations report 
>altimeter. Is that station pressure? So if I wanted to reduce these 
>altimeter reports to sea level I can use the variable PMSL in sflist to 
>get all metar stations to report pmsl?
>And finally, what is the difference between PALT and ALTM?
>Thanks for any help!
>
>
>-- 
>
>James Correia Jr
>--------------------------------------------------
>PhD candidate Ag Met
>Iowa State University
>phone: 515-294-5587
>email: address@hidden
>http://www.mesoscale.iastate.edu/jimmyc/index.html
>--------------------------------------------------
>
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>--the other Homer
>
>"I have no special talent. I am only passionately curious."
>--Einstein
>
>