Will the real pressure please stand up?
What is barometric pressure?
Let's start the ball rolling with the US National Weather Service (NWS) definition: of station pressure:
“STATION PRESSURE: This is the pressure that is observed at a specific elevation and is the true barometric pressure of a location.”
The National Weather Service (NWS)defines station pressure as the true barometric pressure of a specific elevation. Station pressure is, of course, not a sea level pressure unless we are actually at sea level elevation. But what do they mean about barometric pressure? What is it?
I think it would be reasonable to conclude that the NWS is referring to barometric pressure as the atmospheric pressure at a specific elevation.
If we want to split hairs, atmospheric pressure is the true pressure at any elevation, but if you measure it with a calibrated weather station barometer, you can call it a station pressure. The use of the term depends on the context. The term “barometric pressure” is used by the NWS in a generic sense — meaning barometric pressure is a pressure reading from a barometer at a specific location/elevation. In other words, — atmospheric pressure.
However, weather station manufacturers somewhat oddly, have the opposite view; barometric pressure = sea level pressure (a corrected barometric pressure?) but that viewpoint causes even more confusion. Which sea level pressure are they referring to? Altimeter setting/QNH? or maybe SLP/QFF?. And why call it “barometric pressure”? Call it what it is — QFF/SLP or QNH/Altimeter (setting). One manufacturer had two conflicting definitions of “barometric pressure”. One definition said barometric pressure is atmospheric pressure, and the other said it was sea level pressure.
We have a number of different contenders for the definition of barometric pressure:
1. barometric pressure = atmospheric pressure
2. barometric pressure = sea level pressure (SLP?)
3. barometric pressure = sea level pressure (Altimeter setting?)
4. barometric pressure = any pressure from a barometer
5. barometric pressure = raw pressure from a barometric sensor
Pick one — any one; they would all be correct depending on the reference you happen to be choosing from.
All of which leads us to yet another confusion of terms:
What is sea level pressure?
At first blush, the answer is obvious it seems — sea level pressure is a station pressure that has been corrected/reduced to sea level pressure factoring in elevation, temperature, humidity, plateau effect and possibly, other factors.
And that would be a 100% correct answer, except there is a problem. There is another 100% correct answer.
There is another sea level pressure that is often forgotten about. And that would be Altimeter (setting) or QNH which is a simpler version of sea level pressure (SLP). Altimeter (setting)/QNH is station pressure that is corrected/reduced to sea level pressure, factoring in only elevation. Unlike for SLP, temperature changes, humidity and other variables are not used to calculate Altimeter setting/QNH.
Some METAR reports are not especially helpful for discriminating between the two different sea level pressures, either. Sea level pressure in a decoded METAR report would indicate SLP as “SLP (sea level pressure).” But for Altimeter (setting), there is no equivalent descriptor and no indication that it is, equally, a sea level pressure just like SLP.
Unravelling the Tower of Babel.
Since the term, “barometric pressure” has conflicting meanings, in order to not confuse the reader, I am inclined to not use “barometric pressure” except to describe it as a reading from a barometric sensor.
Sea level pressure is also a confusing term in that it can refer to SLP or Altimeter (setting) or both depending on the context. Although SLP and Altimeter (setting) are both sea level pressures, they are not the same. They use different parameters and have different algorithms.
Something new or relatively new — at least for North American weather enthusiasts. I am seeing the term “air pressure” being used more frequently. “Air pressure” is more commonly used in Europe where it means atmospheric pressure. Usually.
To ease the confusion of terms, I have decided to use these definitions:
1. barometric pressure = pressure from a barometric sensor.
2. sea level pressure = SLP or Altimeter setting (tell the reader which one you are referring to!)
3. air pressure = atmospheric pressure at a specific elevation.
4. barometer = instrument used to measure pressure.
Sure, there are regional biases or country specific preferences to describe “pressure”. Even weather equipment manufacturers have their own definitions.
How about the term “Barometer”? The Fine Offset consoles that I have, show “Barometer reading” or just “Pressure” which alternates between Station pressure (ABS) and sea level pressure (REL) depending on which one, you want to display.
A popular data logging software, WeeWX also uses the term “Barometer” and if you are new to this software package, you have to dig deep into the documentation to find out what pressure the program is referring to. What WeeWX calls “Barometer” is actually SLP.
Maybe Ambient/Ecowitt/re-brands(clones) consoles do a better job? Nope. They use a ABS/REL system.(Absolute value and Relative value). There is nothing wrong with using the term “Absolute value” as it is commonly used by barometric sensor manufacturers, but it is unclear what the REL (REL value) is supposed to be set to. Again, there are references to set REL to the sea level pressure at a close-by airport, leaving the owner wondering which sea level pressure to choose.
It seems some (most?) weather station manufacturers are avoiding using the standard meteorological terms like station pressure, Altimeter (setting) or SLP.
Pressure? Which one?
Confusion? Not possible. After all, meteorology is an exact science.
