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Barometer calibration cautions with METAR, Mesowest and NWS


Anyone with a weather station would at some point normally calibrate or re-calibrate their barometers using a close-by airport as their calibrated reference. The given advice is to always converts all pressure readings to hPa (metric) for calibration purposes.

For instance, for Davis and Fine Offset weather stations one would calibrate to the Altimeter (setting) at the airport so naturally, it is important for the calibrated referece to have the correct readings.

Although aviationweather.gov METAR reports provide both Imperial and metric units in their METAR, the Altimeter metric reading seems a bit off. Not by very much but significant enough if you are trying to match baro readings in hPa or mb. You can check for conversion errors (inHg to hPa) by using any number of online conversion calculators.

Here in Canada, we use both Imperial and metric units for METAR. Altimeter is always expressed in inHg and Station pressure and SLP is always in metric.

In the U.S,  pressures are usually in English units. However for Mesowest and NWS you can can optionally change all reporting to be either Imperial/English units or metric and therin lies the problem - conversion errors.. sometimes up to 0.6 mb ... too much to ignore for barometric calibration purposes.

Now the strange thing is that SLP seems totally unaffected by this issue. It is  correct either in Imperial units or metric units! Only Station pressure and Altimeter have this odd conversion error issue.

Therefore,if you use aviationweather.gov, Mesowest or NWS you may want to compare/verify those readings with either the source AWOS operator or your own government weather service. In Canada, this is more difficult than I would like because hourly pressure data seems to be available only in XML format. You have to visually "parse" through the data to get the pressure data you want.

Weatherstats.ca does provides hourly human readable station pressure and SLP but not Altimeter.

If you live in Canada, I would obtain the very latest data from the XML doc and do your own conversions to metric.

The chain of command is: AWOS station operator (i.e  NavCan) ==> ECC ==> NWS or Mesowest or other data "ingestor".

UPDATE: For Canadian weather watchers, here is the link for what appears to be the source data for Canadian AWOS stations. Thanks to weatherstats.ca for providing it.


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