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I agree with all Dave Walden has written about this, but have questions
about his plot of errors in the various methods of predicting refraction.
What were those plots of refraction differences actually differences FROM?

You can only determine such differences if you have some measure which you
know to be absolutely true. In the case of refraction, I know of  no such
standard.

Refraction can be calculated from a multi-layer computer simulation, in
which the different layers have different rates of change of temperature. No
simulation is better than the data that is fed into it, and depends on the
validity of the assumptions that it's based on. For refraction, that depends
on the way in which temperature is presumed to change with altitude, and on
humidity. Actually, humidity has remarkably little effect; it's mostly
temperature. And various "standard" models of temperature variation have
been adopted over the years, to represent some sort of average measured
value, but there's nothing God-given about any of it.

And the other big assumption is that the atmosphere is uniformly layered.
You only need to look at the sky on a day like today, in Oxfordshire, to see
what a dodgy assumption that must be, in practice. About half of the sky is
deep blue, the other is made up of great stacks of cumulus cloud, with warm
air being drawn up from below at the centre, and cold air returning around
the margins. As the clouds pass, those downdraughts can be clearly felt at
ground level; they are real! The notion of uniform layering also belies the
passage of fronts, those wedges of air at differing temperature that drive
much of our weather.

Nevertheless, these are the assumptions that have been built into the
refraction models, on the basis of "what better can you do?". Which is fair
enough. Having made those assumptions, refraction can be computed as
accurately as you wish, but those calculations are never better than the
data that was fed in, and it's as well not to get too dazzled by their
apparent precision. Differing assumptions make hardly any difference at high
altitudes, but can be important near the horizon.

So what Dave's plots show (I suggest) is not absolute error in any
refraction formula, but just how well they conform with some "standard"
model, unspecified.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.




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