Barometer Setting

[scott71]

Ok this is going to sound strange to some but being a new guy at this it's just something I need to know. My question is how do you get your barometer set for your area? I know my local airport will be different from my home due to elevation & distance. And checking with Weatherunderground for someone that is close to me still not at my elevation but only maybe 7 miles as the crow flies away. So far that is how I have it set to, but after looking in to some of that stations readings they seem to be off with certain things. So is there a way to figure this out to get it correct for my weather station's location. My elevation is 548ft above sea level. As of right now 11.20am my barometer is reading 28.87 with rain in the area as a low is moving through. The local airport is showing 29.28 and falling, their elevation is 341ft. I know I'm being picky but when I do something I like to be have it done right, it's just in my nature of the way I do things. Any help would be greatly appreciated to figure this out.

[sam2004gp]

I do mine according to this formula, and check it about once a month.   I too can not trust my local airport.

http://www.csgnetwork.com/barcorrectthcalc.html

[wxtech]

I set my baro by interpolating between 3 NWS stations about 75 miles away (west, northeast, southeast).  Then fine tuned the setting after CWOP quality control evaluated my baro data.  http://weather.gladstonefamily.net/
My setting is based on corrected to sea level data, therefore elevation is not an issue.

[SlowModem]

It is advisable to wait until the pressure is steady or almost steady.

[Bushman]

Doesn;t VPLive do this as well?

[scott71]

Thanks guys for the replies & links!!! Very useful information.

[KeithBC]

Barometric pressure is always reported as a sea-level equivalent.  In effect, it is what the pressure would be at the bottom of a well drilled from the surface of the earth down to sea level. 

A barometer straight from the factory will read the actual air pressure.  You have to correct it for your altitude to get the proper barometric pressure.  If your barometer is corrected for your altitude and another nearby barometer at a different elevation is corrected for its altitude, they will both read the same.

If the pressure is steady, you can take the airport barometric pressure as being yours without worrying about the difference in elevation, as they both should be reporting sea level pressures.  The only correction you would need to make is for any pressure gradient between you and the airport.  Check on a weather chart that shows isobars.  Using the most recent chart, locate the airport and your position on the map and determine from the isobars how many millibars (or millimetres) difference in pressure there is.  Then apply that to the airport's reading to get your reading.

[George Richardson]

Keith,
Thanks for the concise post on barometric pressure readings. I hate to muddy the water but I must. Many people with Personal Weather Stations seek guidance from CWOP especially with its Positive (and Negative) feedback. CWOP uses Altimeter pressure rather than Sea Level Pressure. The small, free program, VPLive actually displays 5 different Barometric pressures, all calculated from the Station pressure. VPLive submits to CWOP the proper pressure readings at the proper times (for Davis VP users). This is especially important at higher elevations. So, just as one location can have several different temperatures readings at the same time (depending on the scale used), that same location has several different Barometric pressures (depending on the scale). But, as you say "Barometric pressure is always (generally) reported as a sea-level equivalent."
FWIW
George

[SlowModem]

CWOP uses Altimeter pressure rather than Sea Level Pressure.

I confirmed this statement in the CWOP guide.  Now I'm confused.  OS and LaCrosse measure SLP.  Seems like this would be a big disconnect.

[KeithBC]

Airport altimeter settings are also corrected to sea level.  I am surprised that you say there is a difference between altimeter setting and barometer setting.  They ought to be the same.  As a former military pilot, I am quite confident that I am correct about the altimeter setting.  I am less so about the barometer setting, though at this moment, I can't see why they would be different.  I will look into that. 

[George Richardson]

Keith,
Steve Hatchett at softwx.com is the man that knows his barometer. His VPLive program is displaying the following "Pressures" for me at this time.

Barometer (Sea Level)   1019.37 mb
Station Pressure            988.59 mb
MADIS Altimeter           1018.29 mb
ASOS Altimeter            1018.69 mb
Sea Level Pressure M    1019.37 mb
Sea Level Pressure U    1019.44 mb

There are postings on this board where the differences are described. There is also information on his website.

HTH

George

[KeithBC]

Thanks George, useful link.  I stand corrected. 

The difference seems to be that the altimeter setting corrects to sea level assuming that all the "air" in that well hole down to sea level is at ICAO standard conditions, whereas the other calculations correct to sea level using different assumptions based on actual surface conditions.  Makes total sense now.

[George Richardson]

Keith,
I'm glad it makes sense to you. I don't know the how or why, just the fact. Apparently, the higher the elevation, the more pronounced the difference is. I'm only at 825'.
George

[killwilly]

I use Google Earth. It will give you Lon and Lat, also altitude.

Alan

[scott71]

Barometric pressure is always reported as a sea-level equivalent.  In effect, it is what the pressure would be at the bottom of a well drilled from the surface of the earth down to sea level. 

A barometer straight from the factory will read the actual air pressure.  You have to correct it for your altitude to get the proper barometric pressure.  If your barometer is corrected for your altitude and another nearby barometer at a different elevation is corrected for its altitude, they will both read the same.

If the pressure is steady, you can take the airport barometric pressure as being yours without worrying about the difference in elevation, as they both should be reporting sea level pressures.  The only correction you would need to make is for any pressure gradient between you and the airport.  Check on a weather chart that shows isobars.  Using the most recent chart, locate the airport and your position on the map and determine from the isobars how many millibars (or millimetres) difference in pressure there is.  Then apply that to the airport's reading to get your reading.

Could you point me in the right direction of a surface weather map that shows isobars. I did a few searches on google but didn't really come up with any current maps for my area (PA). If you have a source map that show current time I'd really appreciate it.

[KeithBC]

Could you point me in the right direction of a surface weather map that shows isobars. I did a few searches on google but didn't really come up with any current maps for my area (PA). If you have a source map that show current time I'd really appreciate it.

Your best bet is something aimed at aviation users.  "Public" weather maps are dumbed down for civilians.  Here is one that I found: http://adds.aviationweather.gov/progs/nav.php?current=0.  It's kind of low-resolution, but it might help you get started.

[scott71]

Thanks Keith!! That was just what I was looking for. Now if I only had access to those "forecaster" grade maps.LOL!!

[KeithBC]

Now if I only had access to those "forecaster" grade maps.LOL!!

I would think that your government would have something available online.  If not, ours does:
http://www.weatheroffice.gc.ca/analysis/index_e.html