wrong pressure in clientraw.txt?

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AenG
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wrong pressure in clientraw.txt?

Post by AenG » Fri Jan 07, 2011 9:27 am

When i change the Station's Altitude from 0 to 3 meter there is no pressure change in the clientraw.txt.

The [actual_thb0_press_hpa] in the allsensors.txt is ok! (pressure goes down)


Is this normal?

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Re: wrong pressure in clientraw.txt?

Post by YJB » Fri Jan 07, 2011 10:05 am

Hi geert,

Are you referring to field 6? It is using [actual_thb0_sealevel_hpa:-].

I don't think that 3 meters should make a noticeable difference this time of the year, since the correction is also based on the current temperature.

Reading at 3 meters height 1015, temperature 0C:
Sea-level corrected pressure: 1015.53

Reading at 3 meters height 1015, temperature 15C:
Sea-level corrected pressure: 1018.03

Reading at 3 meters height 1015, temperature 30C:
Sealevel corrected pressure: 1020.51

Edit
I'm not sure if Boris has taken the latitude into account, since it makes a marginal difference, better to wait until it's summer and start experimenting again.

Edit-2:
Doing a bit of research it seems that humidity is also playing a role AND that there are several ways of calculating pressure at sealevel, each with thier own result.

And to be technically correct, I've used a latitude of 52 for these calculations

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Re: wrong pressure in clientraw.txt?

Post by AenG » Fri Jan 07, 2011 3:50 pm

Yes, i referring to field6.(thats sealevel_hpa)

The difference between 0 and 3 mtr is 0.4 hPa.

Why display sealevel pressure on your website when you live on a hill (2400mtr high)?
Perhaps it is more logical to use [actuel_thb0_hpa] in field 6?

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Re: wrong pressure in clientraw.txt?

Post by YJB » Fri Jan 07, 2011 4:00 pm

Hi Geert,

It has been worldwide agreed that all weather stations are normalizing station pressure to the pressure at sealevel. This is the only way one can compare pressure differences between stations. All weather maps are also showing normalized pressure.
No matter what weather systems are doing, the air's pressure decreases with height. If you're trying to draw a weather map of air pressure patterns, you need a way to remove the effects of the station's elevation. That is, you want to see what the pressure would be at the station if it were at sea level. Otherwise, all high-elevation locations would be mapped as having low pressure.

You need to calculate, sea-level pressure, which is defined as: "A pressure value obtained by the theoretical reduction of barometric pressure to sea level. Where the Earth's surface is above sea level, it is assumed that the atmosphere extends to sea level below the station and that the properties of that hypothetical atmosphere are related to conditions observed at the station."

To do this, you have to take into account the barometric reading at the station, the elevation above sea level, and the temperature.
But, if your really want to deviate from the standard (like my daughter has been saying many times "why was I not involved in the discussion when something important was decided", you can change the /home/meteohub/clientraw.conf file. Be aware that you will need to make the change again after every update.

Cheers,
Ysbrand

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Re: wrong pressure in clientraw.txt?

Post by AenG » Fri Jan 07, 2011 4:03 pm

So, when i understand you right. It is better to change the Station's Altitude to 0 meter?

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Re: wrong pressure in clientraw.txt?

Post by YJB » Fri Jan 07, 2011 4:26 pm

No,

The stations altitude should be reflecting the real altitude. Based on the pressure and the real altitude the station and all software should calculate the "normalized" pressure at sealevel.

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