Think of a 2300 m deep well drilled in the ground with Mexico City being
the well site; the well is lined with steel so water, etc. can't get in it
and it is dry at the bottom; you put the barometer at the bottom of this
well and read its absolute pressure. This is how the standardized
pressure published for Mexico City is determined. The weather bureau
"drills" this imaginary well and assumes that the pressure changes with
altitude in accordance with an international standard arrived at via
averaging data taken over several years. The purpose of standardizing
pressures read from different geographical locations is to detect weather
systems, arrange matters so that at a given site most readings will be
between the high and low ends of the barometer's scale, etc.

On 16 Feb 2005, Rik O'Shea wrote:

Hello,
my understanding is that air pressure [mb] or [hPa] (metric units
please) descreases with altitude. A rough rule of thumb that I've seen
cited is that the correction is 1 millibar for each 8 meters of
altitude gain.

Looking at the weather for Mixico city in the links below I see an
Air-Pressure 1011.2 hPa or Millibar.

I would have though that the measured air-pressure in Mexico city
(2300 m) would
be of the order of ~ 800 hPa. Can someone enlighten me on this issue ?

Thanks & regards

http://www.wetter.com/home/cooperati...18 2&type=WMO

http://www.bbc.co.uk/weather/5day.shtml?world=0300