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