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From: George Huxtable (no email)
Date: Thu Jun 15 2006 - 19:53:53 EDT
Alex mentioned the paper I recommended-
> "I've just received an offprint of a new article by Andrew T Young,
of
| > the Astronomy Deparment, San Diego State University,
"Understanding
| > Astronomical Refraction", which has recently appeared in the
journal
| > "The Observatory"(Vol. 126, no. 1191, pp. 82-115, 2006 April.)"
and asked-
| Have you seen the paper? Is it available on the web?
Yes, I've kindly been sent a reprint. I must be on his refraction
mailing-list, having discussed a lot of details about refraction with
him in the past. I don't know whether it's on the web. I can no longer
find Andy Young's email address at SDSU, but you could try asking
them. I have always found him to be a most helpful character.
In my opinion, his paper is the sort of thing you might want to keep
in printed form, rather than as web ephemera, but I take a somewhat
old-fashioned attitude toward such things. I get the picture that to
some (here I exclude Alex) if it isn't available online then it
doesn't truly exist.
Anyway, now I consider myself somewhat better informed by Andy's lucid
exposition, and can try to comment further about Alex's problems with
dip; if dip really is the underlying reason for his sextant
discrepancies.
Imagine that in the Kielefjord, on the day Alex was observing, there
was a temperature inversion in the air over the surface of the water.
Here we are considering just the lower few feet, between the level of
the water surface and Alex's height of eye; probably just the lower
couple of metres, depending on Alex's height and how far up the beach
he was standing. If in that region the temperature gradient, with
increasing height, was as great as -0.115 degrees C per metre, that is
sufficient to bend light downwards, towards the water surface, so that
it's curvature exactly matches the curvature of the surface. In that
case, light would be "trapped" into following the water surface. In
that case the visible horizon, the boundary between sea and sky, would
appear to be exactly horizontal, no matter what your height of eye. So
the actual dip under thise conditions would not be the text-book value
that Alex took corresponding to his height of eye, but zero instead.
Wouldn't that, on its own, account for most of Alex's observed
discrepancy? If the gradient were higher still, that would give rise
to a reversed dip.
Note that we are talking here about the temperature at the water
surface being only a quarter-degree or so cooler that it is at eye
level, which doesn't seem to be a great deal. However, that gradient
is a lot greater ( and in the opposite direction) than the value taken
for the Standard Atmosphere, which is only +.0065 degrees C per metre.
But there's nothing unphysical or unfeasible about a gradient of -
0.115 degrees C per metre. If the air is cooler below, as it is in
such an inversion, then that is a stable state of affairs, and air
convection doesn't act to stir things up. So, according to Young,
there's no limit to the gradient in such inversions, and "... rates
exceeding a degree a meter are common. An inversion gradient of 20
degrees per metre has been measured directly ..."
So how can such a temperature inversion near sea-level come about?
Consider a land-mass near the water, such as happens in the Red Sea
(and the Keilefjord). The worst situation is apparently caused over
desert sand, and you can sea why. When the Sun shines directly on
sand, it can get so hot that it's painful to walk on, the reason being
that all those grains separated by air, just making point contact with
grains below, act as a good insulator, so heat can't conduct down into
the earth. The high local temperature, close to the surface, causes
the air layer in contct with it to be efficiently heated. Conversely,
at night, the surface of sand cools down very quickly. Black volcanic
sands would presumably absorb Sun energy even more effectively and
heat the air above them more.
But it's not necessary to invoke desert sands. Any land surface will
heat more quickly in daytime, and cool more quickly at night, than the
local sea. In the sea, turbulence causes mixing between the upper
layers, making any water-mass an effective heat-sink, with a
temperature that changes little, and slowly.
Now we have a picture, of air being warmed in the daytime over
adjacent land, then a light breeze carrying it or drifting it over the
surface of the cooler water, so that lower layers of the air, in
contact with that water, are somewhat cooler than the rest, and the
resulting temperature gradient gives rise to anomalous dip. Alex
reports his measurements as being in fine weather, daytime, taken over
a sea-body that's surrounded by land. That seems like perfect
conditions for upsetting the dip. The moral might be that sextant
observations should be taken, not near land, but out at sea, where
there's no local source of warm air.
Does any of that seem plausible? Please note that I am no
atmospheric-scientist, but just doing my best to make a few logical
deductions from the evidence that Young has provided.
George
contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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