[wxqc] Barometric Pressure Errors

[Dave Helms]

Hi Geogory,

*  Altimeter  is determined by using a simple pressure correction to sea 
level using station elevation alone.
*  Mean Sea Level Pressure (MSLP) is determined by pressure correction 
to sea level using station elevation and temperature. 
MSLP is theoretically a more accurate pressure reduction since 
temperature is a major control on air density, and air density controls 
pressure.  Dew point/RH is not included in MSLP calculation, although it 
too influences air density/pressure (re: example of the Marfa, Texas 
"dry line" front where hot dry air forces warm moist air aloft causing 
severe thunderstorms). 

There are several different ways one can use a temperature correction to 
estimate MSLP.  This web page provides some interesting examples:
http://www.crh.noaa.gov/unr/?n=mslp

ln (Psl) = ln (Pst) + (g / ( RTm)) Zst
(Psl = sea-leval pressure, Pst = station pressure, g = gravity, R = air gas constant, Tm = mean temperature
from Zst to Z0 (sea level), Zst = station elevation.  Units:  J, m, K, kg, s.
http://cmsharpe.com/software/cms/cmsWxCalc/cmsWxCalc6/WxCalc6.py

CWOP asks its members to send Altimeter pressure in their weather reports.  NOAA can use your current and 12 hour temperature 
observations to calculate MSLP and station pressure and station elevation.


Hope this helps,

Dave
CW0351


Geoffrey Dick wrote:

>On Mon, 17 Jul 2006 10:03:06 -0400
>"Thomas E. Salzer" <tom.es at verizon.net> said 
>regarding barometric pressure errors:
>
>  
>
>>.... The difference is how one makes corrections for altitude. 
>>If one simply uses elevation, they will get a different result 
>>than if dew point and temperature are included. ...
>>    
>>
>
>I would like to understand the simple physics of how dew point and 
>temperature can affect barometric pressure at a fixed elevation.   
>Can anyone explain it in layman terms?
>
>
>Geoffrey Dick
>
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