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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Extreme dip; was dip angle refraction correction
From: Marcel Tschudin
Date: 2011 Dec 3, 13:06 +0200
John, I don't think that your question was so "esoteric". Thank you for reminding me Andy Young's calculator which could possibly also be used to calculate an extreme dip which came recently to my attention and which in this context could possibly be of a general interest.

Greg Rudzinski informed me off-list on a photo observation of the sun while Santa Ana was blowing which led to an intercept way out (4 nm). It would be interesting to find out the lapse rate which caused this extreme dip. This could be done by finding out which dip would have given a zero intercept and using then the calculator to find out which lapse rate this corresponds to. This lapse rate could then further be compared to measurements from radiosonde measurements. The nearest station to Greg's location is probably Vandenberg AFB (72393). You can obtain their measurements for the date of observation at 00Z and 12Z from this link:
http://weather.uwyo.edu/upperair/sounding.html
Select the time period of interest, enter then their station code 72393 and press <enter-key>. The data appear in a separate window. According to Greg's mail his observation must have been a month ago at the beginning of November. The sounding data of Nov 2 12Z and Nov 3 12Z show indeed very strong inversions.

Marcel

On Sat, Dec 3, 2011 at 12:35 AM, Apache Runner wrote:
I have a somewhat esoteric question, but I thought I'd pose it to the list to see if anyone knew of a program or website that could do this.

Here's the situation - some students are doing a final project where they went to the tallest building in Boston and used a water level to establish a horizontal and then sighted down to the horizon.   They then had measurements of the height of the building and want to turn this into a curvature of the earth.

They'd like to improve on things and correct for refraction.    I don't know of any ready made formulas for correcting for the shift in the position of the horizon due to refraction, since you'd need some model of temperatures etc.

I suggested that they could make a 'model' atmosphere that had maybe 4 layers from the surface to the top of the building and then assume either a uniform temperature or a mild uniform gradient and then just do the ray tracing.   I'd imagine this is a bit of pain, but not outside of their capabilities.

On a related note, since I talked about this effect, they keep asking me about whether they need to correct their dip angle for refraction and it takes a bit of time and patience to show them the relative size of the two effects for an observer only a few feet above sea level.

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