NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Extremely poor conditions??
From: John Huth
Date: 2012 Mar 20, 14:57 +0100
From: John Huth
Date: 2012 Mar 20, 14:57 +0100
In the extreme case of fata morganas, you can get a lot more than 7' distortion. I was kayaking up in Maine, where the water temps were in the low 50's (F) and air temps were in the 90's. We had some rather remarkable distortions.
I think the biggest issue to keep in mind is the relative water and air temperatures. When the difference gets large (> 15-20 deg. F, say), then you start to get strong refractive effects. During a period of large atmospheric temperature fluctuations, I took a series of photos of an island that was close to my horizon. The different images associated with changes in air temperature was striking!
On the other hand, in the same spot, when the air and water temperatures were close to being equal, I had a precision of about 1'.
It's partly a matter of experience and going through what you just went through. Books only capture a fraction of the art, so getting out and taking sightings and working it through under a lot of conditions is the best way to learn.
--
Keeping up with the grind
On Tue, Mar 20, 2012 at 2:31 PM, Alexandre E Eremenko <eremenko@math.purdue.edu> wrote:
Franks,
Thank you very much.
1. Let me bring to your attention that it is the end of March today,
and all messages in our archive for 2012 are still listed under January.
2. The surface water temperature 3 days later was 40 F (I don't know where
to find the temperature for March 17. We were observing from the part of
the jetty which is adjacent to the shore. Bill walked to the end of the jetty, (it protrudes from the shore to the lake) and says it was chilly
on the lake end.
So you mean that the layer of cold air near the water surface acts
like a sort of mirror, and the ray can be bent in the opposite direction
to the usual one by as much as 7'.
Most CelNav books do not warn about this.
Is there a way for a navigator who does not know his exact
position to figure out that such thing is happening?
Well, I sort of remember some discussion of this on the "old list"
under "Anomalous refraction", let me try to find it.
We should have tried the artificial horizon which we had with us!
Alex.
On Tue, 20 Mar 2012, Frank Reed wrote:
Hello Alex, you wrote:
"Bill promises to find the water temperature of water on this day,
but it is really hard to believe in this kind of abnormal refraction. "
I think this is exactly what you were seeing. The lake water is still very cold and then we had this sudden "heat wave". Seriously: 80 degrees Fahrenheit on St. Patricks Day in Chicago?? There have been afternoon and evening fog banks on the lake shore, dense and cold. The conditions that produce this dense, cold, low-hanging fog are notorious for abnormal refraction, too. I think you have seen a really fine example of this!
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Keeping up with the grind
