Topic: Absolute versus Relative Pressure--yawn!

[Saraband]

I know the subject has probably been discussed previously but I'm new to this.  The manual that came with my weather station says I need to calibrate the relative pressure readings of my unit to an official site near me. They suggest going online and doing a search.  I did and found one other PWS in my area and set my unit accordingly.  However, this begs the question: What if there was no other station near me? How would I determine the relative pressure?  How did the lone PWS in my area determine the relative pressure for his unit? I am located about 30 miles from the ocean at an altitude of 3700ft.  Is there a formula to determine relative pressure based the absolute pressure/altitude/temp and/or whatever?  Please enlighten me.

[DanS]

Local TV, radio station weather report?  Hilo Int'l airport might be a source. Mt. View Airstrip seems to be using them.

[nincehelser]

Here's the formula for calculating altimeter from absolute pressure and elevation. 

http://www.srh.noaa.gov/images/epz/wxcalc/altimeterSetting.pdf

Another way...

Acurite gear has a "learning period" where they calculate the average pressure over 33 days, then scale that against the world-wide average pressure of 29.92" as set by definition.  It works pretty well for me and gets within a few hundredths of the local airport, but it's a horribly slow way to set a barometer.  It requires no outside information, though, and it's close enough for jazz.

[George Richardson]

Try http://www.starpath.com/barometers/baro_cal.php

FWIW, I wouldn't calibrate my barometer against another PWS on a dare. Very few people understand barometric pressure and most couldn't care less whether their data is accurate or not just so long as they're on wunderground with Rapid Fire and on CWOP getting green checks.

JMO

George

[nincehelser]

Try http://www.starpath.com/barometers/baro_cal.php

FWIW, I wouldn't calibrate my barometer against another PWS on a dare. Very few people understand barometric pressure and most couldn't care less whether their data is accurate or not just so long as they're on wunderground with Rapid Fire and on CWOP getting green checks.

JMO

George

Well said.  A good station operator should be able to verify the correctness of his own data without comparing notes with others. 

That's what separates the men from the boys.

[CW2274]

Try http://www.starpath.com/barometers/baro_cal.php

FWIW, I wouldn't calibrate my barometer against another PWS on a dare. Very few people understand barometric pressure and most couldn't care less whether their data is accurate or not just so long as they're on wunderground with Rapid Fire and on CWOP getting green checks.

JMO

George

Ohhh, snap!

[Old Tele man]

Yep! Never, "knowingly" send bad data...which implies you KNOW what you (and your wx station) are doing (wink,wink)!

[Saraband]

Thanks for the quick responses.  I'll give the Star Path calculator and the NOAA formula a try.  Off hand I think the NOAA formula would probably give me the best answer however the math seems rather daunting (I'm a liberal arts grad), but it also offers to be a good brain  exercise which is always something I can use.  I am also trying to contact the University of Hawaii Meteorology Department to see if they may have a good idea.

[nincehelser]

Thanks for the quick responses.  I'll give the Star Path calculator and the NOAA formula a try.  Off hand I think the NOAA formula would probably give me the best answer however the math seems rather daunting (I'm a liberal arts grad), but it also offers to be a good brain  exercise which is always something I can use.  I am also trying to contact the University of Hawaii Meteorology Department to see if they may have a good idea.

The math in that formula isn't as hard as it looks.  If you look closely, some of those numbers could be simplified.  I think they just chose to express the formula in an expanded form so that it matches the form of a more general equation.