Author Topic: Barometric pressure change rates  (Read 5058 times)

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Offline Xymox

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Barometric pressure change rates
« on: October 12, 2009, 05:45:55 PM »

I am writing some software for Crestron automation systems to interface to ASOS systems via new XML servers.
http://www3.amss.nws.noaa.gov/

In translating technical weather data to friendly data, I have found a difficult question to answer. I have googled a lot and you would think it would be easy to find.. But nooooo...

What is considered a high rate of change, in mB for barometric pressure ? I want to set a alert to a high rate of change and I dont know what it would be ? The ASOS systems produce mB/3hr change rates. I need to know what is considered high, or worth noticing. 3+ 5+ ?

Also is the any "standard" for the indicators seen on most barometers for "good" "change" and "Stormy"  ?? Again I need the transistion points for each state in mB.

Offline Wx4U

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Re: Barometric pressure change rates
« Reply #1 on: October 12, 2009, 09:21:15 PM »
The closest answer I can recall is defined in the following:

http://www.srh.weather.gov/srh/jetstream/index.htm

Pressure Falling Rapidly
A decrease in station pressure at a rate of 0.06" of mercury or more per hour which totals 0.02" or more.

Pressure Rising Rapidly
An increase in station pressure at a rate of 0.06" of mercury or more per hour which totals 0.02" or more.

Now convert it to millibars... #-o

I'll have to think about your other ???
Retired USAF Air Traffic Controller, Davis VP2 Wireless, WLS 5.8.2, LaCrosse 2308, Logitech Pro 9000 (Roof mounted on rotor), CoCoRaSh gauge, KTXARLIN35, Compaq Presario W/Vista

Offline Wx4U

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Re: Barometric pressure change rates
« Reply #2 on: October 12, 2009, 09:31:38 PM »
This is "The Authority" for US weather observations:

http://www.ofcm.gov/fmh-1/pdf/FMH1.pdf

See chapter 11 for pressure. :-)
Retired USAF Air Traffic Controller, Davis VP2 Wireless, WLS 5.8.2, LaCrosse 2308, Logitech Pro 9000 (Roof mounted on rotor), CoCoRaSh gauge, KTXARLIN35, Compaq Presario W/Vista

Offline Wx4U

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Re: Barometric pressure change rates
« Reply #3 on: October 12, 2009, 10:32:44 PM »
I have looked for a standard for your last question concerning weather conditions. Thus far, I have not found a reference. I have seen and have some very old aneroid barometers.  From 970 to1040 mb (28.64 to 30.71 inches of mercury) scale has been divided into Stormy, Rain, Change, Fair and Dry. There is nothing absolute about associating conditions and values. In certain areas of the world conditions will vary a lot from location to location for the same pressure. Some of the makers of weather station do have a formula they use to present a general forecast in the broadest sense. Trying to develop a program for use in something like ASOS would be more appropriately modeled on factual parameters published by NOAA. Good Luck :!:
Retired USAF Air Traffic Controller, Davis VP2 Wireless, WLS 5.8.2, LaCrosse 2308, Logitech Pro 9000 (Roof mounted on rotor), CoCoRaSh gauge, KTXARLIN35, Compaq Presario W/Vista

Offline Xymox

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Re: Barometric pressure change rates
« Reply #4 on: October 13, 2009, 03:02:57 AM »

Thanks :)

Im trying to translate the technical mB into "Stormy, Rain, Change, Fair and Dry" as you mention.. This is for mostly home use. The ASOS 5 minute files are now available via FTP for most stations so I have written code to pull these and display the information. I wanted to put a color background behind the mB reading that was color coded to the values above. But I need to know how to arrive at these numbers..

Maybe its a + - from a average from your location ? maybe its a devation over 24hrs ?

Maybe rete of change is a better way ??

I just cant find any info on how ALL the makers of barometer come up with the markings !

Kinda amazing that these are marked going way back in time, yet there seems to be nothing as guidence for these markings ?!?!

Offline Wx4U

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Re: Barometric pressure change rates
« Reply #5 on: October 13, 2009, 09:47:01 PM »
If all you want to do is color code the conditions corresponding to the pressure just divide the 70mb difference (1040-970=70).
Now, divide 70 by 5 the number of conditions This would equal 14mb per condition. That seems to work for the 20+ barometers in my collection.

Thus,  970 to 984  = Stormy
          984 to 998   = Rain
          998 to 1012 = Change
         1012 to 1026 = Fair
         1026 to 1040 = Dry

I wouldn't worry about a reference as the procedure has been around hundreds of years and accepted practice.

Retired USAF Air Traffic Controller, Davis VP2 Wireless, WLS 5.8.2, LaCrosse 2308, Logitech Pro 9000 (Roof mounted on rotor), CoCoRaSh gauge, KTXARLIN35, Compaq Presario W/Vista