Meteobridge and SLP vs. altimeter pressure
Moderator: Mattk
Meteobridge and SLP vs. altimeter pressure
I downloaded a table of altimeter and SLP pressure readings from a nearby airport and compared them to the values reported from my station by meteobridge to CWOP and WU. If I am correct meteobridge reports altimeter pressure to CWOP and SLP to WU. The problem is that the offset for each is different, and in the opposite direction. When compared to my local airport's altimeter readings, my station's reports average error is -0.9 mb. When compared to the same airport's SLP readings my station's average error is +0.7 mb. Obviously it can't be that my sensor is too high and too low at the same time. This points to differences in how NOAA is calculating pressures and how meteobridge is calculating pressures. Is there some way to investigate the issue to make sure there is not an error occurring?
Re: Meteobridge and SLP vs. altimeter pressure
LAWx,
Read your post with interest. The Barometric questions has been an obsession of mine and an anal one at that. From what I gather, MB does a pretty good job at calculating Station pressure to Altimeter, providing your station (raw) pressure is correct. The Sea Level Pressure is another story. I cannot figure out what MB uses as formula for that conversion. I have written Boris numerous time on the subject, only to hit dead air. The Altimeter pressure is not a linear formula and can be googled easily. There are really two variations of this formula. One which is the Official one which includes the 10 foot offset, and the other does not include the offset.
The SLP calculations is very sensitive when you go higher in station altitude. At sea level, pretty much the same. At mid altitudes, its a roller coaster ride. There are many formulas for SLP, and they vary per agency that collect them and publish them. If you are trying to match the calculations you see on the NOAA METAR site of your airport, good luck. The NOAA/ASOS uses reduction factors that are site specific. There are other long hand equations that also calculate SLP for anywhere based on 12 Hour avg temperature, dewpoint-RH, elevation and gravity (better brush up on your math skills). A quick read of the Manual of Barometry will indicate the complexity of the calculations and depending of which parts of the world you live in, the values for constants will differ. The error and variations of these different equations will get you close, but no cigar, insofar as matching METAR/ASOS numbers. One needs to consider also the instrument errors, temperature drifts, environmental conditions around the sensor and sitting. Unless you are using a triple class A sensor, your margin of error will not be pretty.
I have asked Boris to include offsets for the SLP, based on the Station Pressure, or ways to modify the formula. The result will at least be closer to reality. I get variations of 30 mb at this point using the existing system, but I am also at 1550 meters elevation.
Read your post with interest. The Barometric questions has been an obsession of mine and an anal one at that. From what I gather, MB does a pretty good job at calculating Station pressure to Altimeter, providing your station (raw) pressure is correct. The Sea Level Pressure is another story. I cannot figure out what MB uses as formula for that conversion. I have written Boris numerous time on the subject, only to hit dead air. The Altimeter pressure is not a linear formula and can be googled easily. There are really two variations of this formula. One which is the Official one which includes the 10 foot offset, and the other does not include the offset.
The SLP calculations is very sensitive when you go higher in station altitude. At sea level, pretty much the same. At mid altitudes, its a roller coaster ride. There are many formulas for SLP, and they vary per agency that collect them and publish them. If you are trying to match the calculations you see on the NOAA METAR site of your airport, good luck. The NOAA/ASOS uses reduction factors that are site specific. There are other long hand equations that also calculate SLP for anywhere based on 12 Hour avg temperature, dewpoint-RH, elevation and gravity (better brush up on your math skills). A quick read of the Manual of Barometry will indicate the complexity of the calculations and depending of which parts of the world you live in, the values for constants will differ. The error and variations of these different equations will get you close, but no cigar, insofar as matching METAR/ASOS numbers. One needs to consider also the instrument errors, temperature drifts, environmental conditions around the sensor and sitting. Unless you are using a triple class A sensor, your margin of error will not be pretty.
I have asked Boris to include offsets for the SLP, based on the Station Pressure, or ways to modify the formula. The result will at least be closer to reality. I get variations of 30 mb at this point using the existing system, but I am also at 1550 meters elevation.