Singer's Creations Forum

pressure relative or absolute?

WWL displays pressure values given by weather station. They can be relative or absolute depending on PWS owner preference. This causes pressure data are inconsistent even for local stations. Now, weather station can show its altitude. The question is: could it be possible to detect if pressure shown is absolute and in that case to convert it to relative value? Then WWL could always show relative pressure value.

Do you have an example of two weather stations currently showing relative and absolute?

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Yes… For instance, IKATOW14 shows currently 981 hPa while IKATOW4 shows 1016 hPa. Distance between these stations is ca. 1 km. So I assume the first station shows absolute and the second relative pressure values.

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Those stations are reporting those values as the “pressure”. Even if they were able to send relative or absolute, there is nothing in the data that reveals that. Also, I’m not sure most (all?) weather stations would be collecting anything other than the absolute pressure. The calculation to relative pressure would need to be done elsewhere after that data is collected – and I’m not sure how useful that would be in most cases since people usually install weather stations to report the data at that exact location (and not at sea level).

Another possibility is that some stations are reporting incorrect pressure data.

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I understand every PWS owner should set his (her) station to relative (sea-level) pressure in order to make data comparison possible (at least so meteorologists say). Clearly it’s not the case (see IKATOW14). In the effect WWL is not able to determine what pressure value (rel or abs) it shows. Best it can do now is to show pressure changes for selected station. So it is impossible to compare pressure values between different stations. Of course it’s not a WWL fault; I’d say it’s a feature which originates at badly configured PWSs due to PWS owner incompetence (or maybe incomplete weather data specification) . Thanks for remarks, Mike.

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I send my weather data to Weather Underground using an Ambient Weather Meteobridge. It has a built in configuration for sending data to Weather Underground (and a bunch of other weather services). While I can see this device is able to send pressure as relative or absolute, I cannot see which is being sent to Weather Underground since it’s preconfigured (likely by whatever Weather Underground asked Ambient Weather to send). And if any weather stations are configured to send the relative pressure, the altitude setting on the weather station could cause an incorrect pressure value if not set properly.

There are a few possible points of failure here.

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AFAICS user manual of WeatherBridge clearly says: “Enter your station altitude in meters. The WeatherBridge will automatically calculate the sea-level corrected pressure (or relative pressure) from the measured pressure (or absolute pressure).”. So you can be sure it sends relative pressure to the Internet as expected.

Most probably IKATOW14 is not set properly.

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