Next message: Frank Reed: "Re: Bubble Horizon Altitude Corrections"
George H wrote:
"What's needed is
a barometer that was set correctly in millibars at sea level somewhere:
then, in a mountain location, its reading falls to the local value."
Yes, George, that is exactly what I was getting at. The problem is that this
is not how barometers are customarily used, so, for example, if you're in
Denver practicing celestial, and you naively use a local barometer or perhaps a
pressure from a local weather report, you will get refraction values that are
too large (in absolute value).
And:
"Except for those at sea level, sea-level pressures and temperatures are
irrelevant. Navigating on the Great Lakes, for example, it's the local
pressure at the level of the lake, not at sea level, that applies."
That would be true if the barometer were calibrated at sea level, but
barometric pressures as reported and customarily used are NOT calibrated for sea
level. They are corrected TO sea level equivalent pressure. You have to take out
that correction (or apply a separate correction based on your altitude above
sea level) to compensate for the common standard for barometric pressure. If I
shoot a celestial sight in Denver and base my altitude corrections on the
barometric pressure reported on the local radio (or a barometer calibrated locally)
the refraction won't be right. At 5000 feet altitude, I estimate that the
refraction values would be too large by about 15%, and that's a big deal. On Lake
Michigan, refraction would be over-estimated by only about 2%
--inconsequential in most celestial navigation sights but worth being aware of.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
