CWOP expects altimeter, gets SLP

[admin]

Looks like there has been a typeo in my imlementation.
old values are:

Code: Select all

actual_thb0_press_hpa 1036.0
actual_thb0_press_psi 15.03
actual_thb0_press_mmhg 777.0
actual_thb0_press_inhg 30.59
actual_thb0_sealevel_hpa 1039.2
actual_thb0_sealevel_psi 15.07
actual_thb0_sealevel_mmhg 779.4
actual_thb0_sealevel_inhg 30.69
actual_thb0_altimeter_hpa 1036.6
actual_thb0_altimeter_psi 15.03
actual_thb0_altimeter_mmhg 777.5
actual_thb0_altimeter_inhg 30.61

new values are:

Code: Select all

actual_thb0_press_hpa 1036.0
actual_thb0_press_psi 15.03
actual_thb0_press_mmhg 777.0
actual_thb0_press_inhg 30.59
actual_thb0_sealevel_hpa 1039.2
actual_thb0_sealevel_psi 15.07
actual_thb0_sealevel_mmhg 779.4
actual_thb0_sealevel_inhg 30.69
actual_thb0_altimeter_hpa 1039.3
actual_thb0_altimeter_psi 15.07
actual_thb0_altimeter_mmhg 779.5
actual_thb0_altimeter_inhg 30.69

[N5XL]

Looks like you got the problem fixed.

I spent today pulling some NWS observations from various locations in North America with a significant spread of altitudes, temperatures and humidity levels. I've posted those observations below. The data in regular text is the observed values reported at each NWS station and the italic text values are values I calculated. You'll want to compare the last columns (orange to orange and green to green) to see the difference between the altimeter formula and the NWS altimeter calculations.



(If you don't see two orange columns and two green columns, the full image is here...for whatever reason I dont always see the full image posted here even after resizing the photo.)
http://i33.photobucket.com/albums/d82/N ... ations.jpg


From what I can tell, the differences between the NWS reported value for altimeter vs what the spreadsheet altimeter formula calculates appears to be due to rounding and the use of significant figures from within the spreadsheet...in other words, this is likely as good as its going to get without a few formula and constants tweeks.

Boris, unless anyone else sees issues with the formula or the results it produces, I think you've got it nailed down.

[N5XL]

Boris,
One final tweak to make things a bit closer

It appears that the United States NOAA (NWS) uses the same Smithsonian derived altimeter formula as we have been discussing with a slight change. Instead of subtracting 0.01 from station pressure, they subtract 0.03...otherwise, the formula is the same. This has the effect of lowering Altimeter values by 0.02 mbar.

Reference is here...(with a nifty calculator to play around with Altimeter if you dont want to create an excel file).
http://www.srh.noaa.gov/epz/?n=wxcalc_altimetersetting

and a pdf copy of the formula here...
http://www.srh.noaa.gov/images/epz/wxca ... etting.pdf

So for those of you still following along...
The NWS derived Excel spreadsheet formula for Altimeter is: NWS Altimeter=(P-0.03)*(1+(((P0^N*a)/T0)*(H/(P-0.03)^N)))^(1/N)

Where:
P is your console pressure in mbar
P0 is 1013.245 (std sea level pressure in mbar)
a is 0.0065 (lapse rate)
T0 is 288 (std sea level temperature)
N is 0.190284 (exponent)
H is the height of the console barometric sensor, in meters

I've updated my excel spreadsheet to have all three Altimeter formulas for those that want to play around...awk code is in there too...feel free to dl from my dropbox.
https://www.dropbox.com/s/pyxfrfyq6x8gi ... 0edit.xlsx

I think this is as far as I can take it.

BR
David (N5XL)