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Re: Refraction
From: George Huxtable
Date: 2008 Jul 4, 11:29 +0100
From: George Huxtable
Date: 2008 Jul 4, 11:29 +0100
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. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---