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According to the Smithsonian Meteorological Tables, one atmosphere is
defined as 1013.250 mb, which the Nautical Almanac approximates as 1010
mb.  One atmosphere corresponds to about 29.92 inches of mercury
depending upon the local force of gravity.  Reducing this by the factor
of 0.9264 quoted by Paul Hirose as appropriate for 2100' elevation
yields a standard pressure at 2100' elevation of 27.72 inches of Hg.  I
approximated these values as 30" Hg = one atmosphere and subtract 2
inches for 2100' elevation, which are fairly easy to remember; the
approximation could be refined for the actual pressure obtained from
the newspaper, tv or internet (drill down from
http://iwin.nws.noaa.gov/iwin/graphicsversion/rbigmain.html into the
political map of the U.S. to one state, and then to one station to find
hourly reports of pressure and temperature).  The approximation might
yield a correction from the bottom of Table A4 correct (compared to
George Huxtable's exact treatment) to within 0.1', perhaps 0.2' of arc.

At one point in this quest I purchased the Smithsonian Meteorological
Tables to try to compute some of the values, but it would take more
study than I have time, patience and skill in physics (thank goodness
that George is with us!).  Mostly I was interested in determining the
appropriate temperature to use, as the formulas employ the mean
temperature of the air column.

I note from page 389 of the Smithsonian Tables that refraction is also
dependent upon the humidity of the air.  This has not been factored
into the corrections, perhaps because the mean humidity of the air
column above a ship could not be determined accurately close to the
sea.  I am tempted to try to estimate the magnitude of the effect.  It
may be one of the factors causing discrepancies in Doug's observations.

I am sure this was all well determined by the astronomers before 1850.
We are treading into astronomy here, although this does lead back to my
quest of checking the calibration of sextants, which service appears to
be unavailable commercially in the United States.

Fred

   

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