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George wrote:

> Refraction works in the opposite direction to geometrical dip, reducing it
> by about 8% when seen from a boat. It can be roughly predicted, for
> standardised atmospheric conditions. However, the lower few feet of the
> air, being strongly influenced by the temperature of the sea surface just
> below it, can suffer from unpredictable temperature gradients, which can
> upset the predicted dip by several arc-minutes, on a bad day. This can be
> the biggest source of error in marine sextant observations.

Robert responds:

Again, this gets back to a handful of questions I earlier asked but to which
no one replied, so I will ask you directly George:

In your experience, are there certain weather conditions and/or other
observable factors during which these unpredictable errors are likely to
occur?


Robert

   

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