Relative and Absolute Air pressure

[watsonm]

Hi,
I have recently configured my setup to send data to Ecowitt.net. When logged on to this site you can pull down a map and see other users data.

What has suprised me is that quite a few datasets have Absolute air pressure greater than Relative pressure. Ok the odd one or two may be below sea level so I am wondering if folks just tweak the Relative pressure offset on the console and totally ignore the Absolute Pressure. (or is there another reason?)

I am using a BMP388 sensor that measures Air pressure (since my console is deceased) and use some Python code to convert the Absolute Pressure given to Relative pressure using the following formula from the referenced web page.

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pressureR = float(pressureA) + ((float(pressureA) * 9.80665 * altitude_amsl)/(287 * (273 + temperature + (altitude_amsl/400))))

See this site for description:

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https://gist.github.com/cubapp/23dd4e91814a995b8ff06f406679abcf

[HansR]

I am not really sure what your question is. Yes, I see a lot of variation (tooo much) on that map but I don't think I can select absolute or relative. I think I just gave altitude of the station and that is it, Ecowitt does the rest. Or am I missing a point?

[watsonm]

On the Pressure panel it usually shows Relative Pressure on the left and Absolute on the right.

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https://www.ecowitt.net/home/index?id=102310

Mine (link above) shows Relative greater than Absolute. A random selection of other sites display Relative lower than Absolute which
believe is unlikely in most cases.

(yours seems to show Absolute and Relative the same so I guess your toes are in the sea!!
being in Holland not so unusual!! )

[philpugh]

I agree with Mike.

Relative Pressure is what the pressure would be at sea level (i.e., Relative to Sea Level); Absolute is the actual pressure you measure.

Usually Relative is greater than Absolute, exceptions being:

1/ Readings taken at mean sea level - both Absolute and Relative should be the same. But by definition you would be under water for some part of the day!

2/ Readings taken at elevations below sea level (e.g., Death Valley) where Absolute will be greater than Relative.

[Mapantz]

My absolute pressure is greater than my relative pressure, which makes no sense. However, there's nothing that can be done about it..

Relative pressure is the one that needs to be adjusted, and if I do that by looking up stations and metar data to get a figure, it'll always be below the absolute pressure reading.

[mcrossley]

The app lets me adjust both absolute and relative pressures independently.

[HansR]

On the Pressure panel it usually shows Relative Pressure on the left and Absolute on the right.

Code: Select all

https://www.ecowitt.net/home/index?id=102310

Mine (link above) shows Relative greater than Absolute. A random selection of other sites display Relative lower than Absolute which
believe is unlikely in most cases.

(yours seems to show Absolute and Relative the same so I guess your toes are in the sea!!
being in Holland not so unusual!! )

Indeed I live at 0 m so that is sealevel (for the time being as its slowly rising )
Actually it should be -1m but I found it easier to have both the same.

[HansR]

The app lets me adjust both absolute and relative pressures independently.

Ah yes, in the calibration section.

[mcrossley]

Though why do they not use the more "normal" terms of station pressure and sea-level pressure.

[HansR]

Though why do they not use the more "normal" terms of station pressure and sea-level pressure.

China... think different!

[access-mdb]

I'm quite close to RAF Benson, so I can compare my relative pressure with theirs. The relative (read Sea Level) pressures are about the same. the absolute (read station) pressures shouldn't match so well as we are at a different heights AMSL. I too notice that other stations on the Ecowitt web page are too low, so it looks like they don't know the difference - perhaps because of the terminology used. There is one not far away which is far too high (faulty pressure sensor?) I assume that the website shows the relative value but people haven't amended theirs. If you think about it, station level pressure isn't very useful on a map - you can't draw isobars when stations have different heights. Of course, station level pressure is used for aircraft landing as their height above the airfield is more important than their height above mean sea level!

[watsonm]

Of course, station level pressure is used for aircraft landing as their height above the airfield is more important than their height above mean sea level!

Just make sure you know your QFE from your QNH

[access-mdb]

And QBA, QBB and QNY are suitable as well!