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If one looks, for example, at the NOAA web site for Denver International Airport we see (at the time of posting this question) the Pressure/Altimeter is 29.78 inHg, the Pressure/sea level is 1006.1 mb and the temperature is 54F. If we assume 1 inHg is equal to 33.8639 mb, then I would have expected 29.78 * 33.8639 = 1008.4 mb for the sea level value. But the NOAA web page lists 1006.1 mb. Why is there (always) a slight difference?

-thanks

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  • $\begingroup$ I'll update the question to current conditions w/temperature @Bianfable. $\endgroup$
    – st2000
    Jan 5 at 19:05
  • $\begingroup$ I understand that American configured aircraft use inHg in the Kollsman window of their altimeters where as foreign configure aircraft use mb in the same. Still don't understand the slight difference. Unless my conversion is incorrect. $\endgroup$
    – st2000
    Jan 5 at 19:15
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    $\begingroup$ The unit is usually written as ‘hPa’ as per SI. ‘mb’ has long been defined to mean the same thing, so it does not matter too much. $\endgroup$
    – Jan Hudec
    Jan 5 at 22:20
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The difference has nothing to do with unit conversion (which is correctly done in your question). The difference comes from temperature: the aircraft altimeter assumes a standard temperature lapse rate of 1.98°C per 1000 ft altitude when converting a pressure to an altitude reading. The reference pressure for the altimeter (QNH) is defined such that the aircraft altimeter will show the correct airport elevation when it sits on the ground.

The sea level pressure however takes the actual temperature into account. If this deviates from the expected value in the standard atmosphere, you will see a different pressure value here. The higher the elevation, the more pronounced will these differences typically be. That's probably why you noticed these differences at Denver. In your example, the temperature was 54°F, which is about 12°C, but the expected temperature at Denver is

$$ \vartheta = 15^\circ \text{C} - 1.98^\circ \text{C} \times \frac{5434 \, \text{ft}}{1000 \, \text{ft}} \approx 4.2^\circ \text{C} $$

Here is the full explanation from NOAA:

ALTIMETER SETTING: This is the pressure reading most commonly heard in radio and television broadcasts. It is not the true barometric pressure at a station. Instead it is the pressure "reduced" to mean sea level using the temperature profile of the "standard" atmosphere, which is representative of average conditions over the United States at 40 degrees north latitude. The altimeter setting is the pressure value to which an aircraft altimeter scale is set so that it will indicate the altitude (above mean sea level) of the aircraft on the ground at the location for which the pressure value was determined. The altimeter setting is an attempt to remove elevation effects from pressure readings using "standard" conditions.

MEAN SEA LEVEL PRESSURE: This is the pressure reading most commonly used by meteorologists to track weather systems at the surface. Like altimeter setting, it is a "reduced" pressure which uses observed conditions rather than "standard" conditions to remove the effects of elevation from pressure readings. This reduction estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station. In practice the temperature used in the reduction is a mean temperature for the preceding twelve hours. Mean sea level pressure should be used with caution at high elevations as temperatures can have a very profound effect on the reduced pressures, sometimes giving rise to fictitious pressure patterns and anomalous mean sea level pressure values.

(NOAA - Pressure Definitions)

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  • $\begingroup$ So, if I find NOAA data on an airport at standard temperature (which I think is 59F) then the conversion should work. I see KIAH was at 59F on Jan 5th @ 9:53am. At that time pressure was recorded at 30.21 inHg and 1022.8. And 30.21 * 33.8639 = 1023.0. That sounds close enough! $\endgroup$
    – st2000
    Jan 5 at 20:01

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