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Jared,
I agree that the effect seems quite large. Bowditch's discussion indicates
that the various attempts to derive a correction factor based on sea-air
temperatures never converged on a solution because of the difficulty of how
and where to make the measurements (sea/air temps at sea level near the
vessel only capture part of the picture).  I don't think I will estimate and
apply this correction on a regular basis in the future. The practical lesson
for me is that I will be more aware of the magnitude of the potential error
and aware of the likely direction of the error.  That is simply one more set
of factors to consider when building a "margin of safety" around my
estimated position.
Arthur


----Original Message Follows----
From: Jared Sherman 
Reply-To: jared.sherman@verizon.net
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Sextant Accuracy and anomalous dip.
Date: Mon, 17 Mar 2003 23:20:23 -0500

Arthur-
  If I understand this correctly then, when the air is warmer than the sea,
the observed altitude is too small. And conversely when the sea is warmer,
the observed altitude is too large.
  But one minute per six degrees F seems like it is big enough so that it
should have been a "routine" correction in sight reductions, rather than
some obscurity to be dug out of an old Bowditch. With a winter ocean near
32F on a cold 5F day...that's an easy 4-5 minute error in the readings!
Ditto for those lazy days when the water is near 70F but the air has been
pushing the high 90's and local heat from a beach seems to be boiling the
air even more.

  It just seems odd that an error of this magnitude would be routinely
ignored.


Arthur Pearson
arthurpearson@hotmail.com

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