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Subject: Re: Table A4 + elevation?
From: Trevor J. Kenchington (Gadus@XXX.XXX)
Date: Fri May 02 2003 - 16:36:15 EDT
George Huxtable wrote, in his (as usual) magisterial comment:
> Nautical Almanac table A4.
>
> Doug says he has used this table to correct for non-standard refractions,
> but I am not sure how he has done so, at his height which (I think I
> remember) he quoted as 2100 ft.
>
> Vaguely remembering (or maybe misremembering?) the density (relative to
> water) of air as 1/830 and of Mercury as 13.6, it seems to me that at
> Doug's height of 2100 ft. the atmospheric pressure will be reduced by 7.5%
> below what it is at ground level. (Somebody please check!) If that's
> correct, then the local pressure will reduce to 935 millibars, compared
> with its "standard" sea-level value of 1010. That takes it well outside the
> realms of the diagram that goes with table A4, which goes no further down
> than 970.
Is Table A4 correcting primarily for refraction in the layers of air
immediately above the observer or primarily for refraction higher up? If
the former, then I fully understand George's point: The lower pressure
at altitude means less refraction and Table A4 doesn't extend far enough
to cope with observations taken from far up a mountain but the equations
from which the Table was drawn should be applied to the air pressure
observed at Doug's location.
However, if the bulk of the refraction occurs much higher in the
atmosphere, air pressure at the observer's location can only give an
approximate indication of the density of the air far above but that
approximation would need to be based on observed surface pressure
standardized to sea level. Indeed, if we are dealing with substantial
refraction high in the atmosphere (even if there is more per metre in
the few metres nearest the Earth), it might be necessary to use a
correction drawn from Table A4 using surface pressure standardized to
sea level and then a second correction based on the pressure difference
between the observer's altitude and sea level to allow for the lack of
low-level refraction resulting from the observer's elevation. The
combination of the two corrections would presumably be intermediate
between what Table A4 gives for the sea-level pressure and what it would
give if the pressure at altitude were treated as a sea-level observation.
I suppose that somebody may have figured out the details of refraction
at high altitudes for use in aircraft navigation but perhaps aircraft
move too quickly for anyone to care about the last 0.1 minutes precision.
Trevor Kenchington
-- Trevor J. Kenchington PhD Gadus@XXX.XXX Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555Science Serving the Fisheries http://home.istar.ca/~gadus
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