[time-nuts] Weather/units question for European members

Graham planophore at aei.ca
Sun May 25 23:42:02 UTC 2014

atmospheric pressure taken for a weather observation is measured in 
millibars and respresents the barometric pressure at ground level at 
that location. If the instrument is located somewhere besides ground 
level it will be corrected to ground level.

The value given in inches of mercury is that barometric pressure 
corrected to sea level and converted to inches of mercury. This is the 
altimeter setting and will be used by aircraft to adjust their altimeter 
so that when on the ground the altimeter will read the height of the 
aircraft above sea level, in other words the ground elevation on which 
the aircraft is sitting. This means there is no direct comparison 
between the pressure given in millibars and that given in inches of 
mercury (altimeter setting) unless the altimeter setting is corrected to 
the same height above sea level at which the atmospheric pressure was 

The Altimeter setting in Europe is not the same.  There the altimeter 
setting is such that the aircraft's altimeter reads ZERO when on the ground.

Two miles away is pretty close but can make a difference as can a 
difference in ground elevation but it sounds like you are taking ground 
elevation into account already.

cheers, Graham ve3gtc

On 2014-05-25 23:06, Mark Sims wrote:
> I ran across this very issue when trying to calibrate my barometer chip against the NWS station located less than two miles away.  Their numbers for millibars and inches of mercury do not agree.  I sent them an email and asked what was going on.  They said their instruments read out in millibars (to three decimal places) The reported value is converted to sea level pressure and reported to two decimal places. They are also converted to inches of mercury for their reports.  Only problem is their conversion constant is NOT the proper value.  They consistently report around 0.02" too high.   I reported this back to them,  but have received no further responses.
> Note that the conversion between true pressure readings and sea level pressure involves an equation with about a fifth power/root (depending upon the direction of the conversion) so it can be quite sensitive to true chip calibration.  The pressure chip that I am using (MP5611) is factory calibrated and has calibration constants stored on-chip (the Bosch BMP085 and BMP180 chips also do this),  but the soldering process can affect the chip so you need to do some final calibration.  The MP5611 can detect the air pressure change seen by raising the chip less than 6 inches...
> Relevance of temperature/humidity/pressure sensors to time-nuttery?  We all know the comparatively massive effects of temperature on our equipment.  But humidity and air pressure also affect them in many subtle and not-so-subtle ways.  I'll post some recommendations/observations on various sensor chips in a while.
> ---------
> One funny thing about weather measurements is that the data that NOAA reports is not what it would seem. The standard ASOS data (which is what NOAA reports in its local current conditions) includes barometric pressure in inches of mercury and in hectoPascals. It turns out that neither is the actual barometric pressure.
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