NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: U.S. Standard Atmosphere 1976
From: Frank Reed CT
Date: 2005 Sep 6, 12:18 EDT
From: Frank Reed CT
Date: 2005 Sep 6, 12:18 EDT
Paul Hirose wrote: "The only temperature vs. latitude graphs I see are for heights of 40, 60, and 80 km. I don't think these are very useful for computing refraction, since above 40 km the air is so rarefied that surely it can't have much effect." It seems that the "US Standard Atmosphere" is primarily intended for aeronautical engineering of high-altitude aircraft and hypersonic re-entry vehicles (there are still some thousands of these in silos and submarines around the world). The troposphere, the portion of the atmosphere which is mixed by active turbulence and extends from ground level up to 10 or 15km, is too messy for such models. But for refraction, the troposphere is 99.5% of the the game. At and close to sea level, the variability with season and latitude makes almost no difference for refraction above 3 degrees altitude, but when you get up to 5 or 10 km above sea level that variability makes a measurable difference but probably nothing of practical significance. With all these small details flying around, it's probably worth mentioning that you get an excellent approximation for refraction at various moderate heights above sea level, e.g. for shooting practice sights in mile-high Denver, by taking the sea level refraction table and multiplying by exp(-height/34,000 feet). At an altitude of 5000 feet, this is a factor of about 86.3%. When you adjust for tempreature and pressure, the air pressure has to be the "sea level" adjusted pressure which is what's ordinarily reported in weather data so that's no problem. But in addition, before making the temperature/pressure adjustment, the temperature should be adjusted to sea level at a rate of about 6.5 degrees C per kilometer of altitude or 3.5 degrees F per thousand feet of altitude. That is, if you're 5000 feet high and the air temperature is 32 F while the barometric pressure is 29.90 inches, this corresponds to a sea level temperature of just about 50 F (and the pressure is already adjusted to sea level) so you're right at the standard conditions for the usual refraction tables --no adjustment necessary. I don't think this second part has been discussed before on the list, and I haven't thought about thoroughly enough to "sign off" on it as correct but I think I've got it right. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars