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» Online Classroom   » Marine Weather   » Public Discussion of Marine Weather   » Barometer calibration: which pressure to use?

Author Topic: Barometer calibration: which pressure to use?
Ron Baker

 - posted December 03, 2015 12:15 PM      Profile for Ron Baker           Edit/Delete Post 
I have a question with respect to the METAR pressure readings at selected stations. All stations seem to include a pressure reading in their METAR data. But some stations also include a second reading labeled ‘SLP’ in the REMARK section. Often the 2 readings are very close or even identical, but sometimes I’ve noticed up to a 1 mb difference between them.

When using the Starpath barometer calibration program, should the pressure reading which is normally present in the METAR data be used even when the station includes the SLP reading in the remark section? Or is it better to use the SLP reading when available? I understand both readings are sea level pressures.

David Burch

 - posted December 03, 2015 04:13 PM      Profile for David Burch           Edit/Delete Post 
It is best to use the one called SLP. Yes, both are referenced to sea level, but the one called "altimeter" uses a formula that is based on the standard atmosphere, whereas the other one "SLP" uses more sophisticated corrections that depend on the history of the air temperature and even special factors for that particular location. They will differ most for higher stations, especially in more extreme temperatures... that is much colder or hotter than expected at that elevation on that date.

Our page at www.starpath.com/barometers includes the temperature factor, so you can experiment with that to get a feeling for the differences in different conditions.

From: Starpath, Seattle, WA
David Burch

 - posted December 03, 2015 04:15 PM      Profile for David Burch           Edit/Delete Post 
PS... i have taken the liberty to change the original title of the post which said "Which station pressure to use" to read simply "which pressure to use" because station pressure is a unique concept on its own (never shown in the station reports!), which is not crucial to this question.

in a day or so, i will remove this note about the change.

From: Starpath, Seattle, WA

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