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    Re: Refraction. was: Bubble Horizon Altitude Corrections
    From: Fred Hebard
    Date: 2004 Jul 12, 00:06 -0400

    OK ladies & gentlemen.  We have the case of a mirage, where the image
    rises above the desert floor.  Are there any counter examples of an
    image sinking below the horizon?
    On Jul 9, 2004, at 3:58 PM, Trevor J. Kenchington wrote:
    > A couple of days ago, I wrote:
    >>> Temperature inversion in the San Fernando Valley, California.
    >>> Bob
    >> I stand to be corrected but I think that is a temperature inversion,
    >> not
    >> a density inversion. I think (but again no more than that) that the
    >> density of air is significantly affected by humidity, as well as
    >> temperature and pressure, such that warm, humid air can be stable
    >> under
    >> colder, drier air.
    >> Time to step aside from his topic and leave it to atmospheric
    >> scientists
    >> -- which I am not!
    > I'm still no atmospheric scientist but, since everyone else has been
    > kind enough to ignore the obvious error in the above exchange, I guess
    > I
    > should point it out. (At least, the error should have been obvious but
    > it took me a day or so to notice it.)
    > The normal pattern of temperatures in the atmosphere is one of
    > decreasing temperature with increasing height (though pressure also
    > decreases with height, such that density falls despite the lower
    > temperature). In a temperature inversion, there is an _increase_ in
    > temperature with altitude and thus a sharper than normal decrease in
    > density -- the exact opposite of Bob's suggestion that density
    > increases
    > with height in an inversion.
    > This sharp drop in density at a temperature inversion can often be seen
    > when small-scale ones form on misty mornings: Smoke will rise from
    > sources near the ground but then flatten out at the height of the
    > inversion. Its density is low, as a result of the high temperatures
    > from
    > whatever fire is making the smoke, but no low enough to penetrate the
    > low-density warm layer above the inversion. Hence the smoke gets
    > trapped
    > near the ground (which, on a much larger scale, is why temperature
    > inversions in the San Fernando Valley lead to smog problems.
    > To return to my original point: I don't think that high density air
    > overlying low density can be stable. George has suggested mirages as
    > examples but those are very local and very unstable (hence the
    > shimmering often seen with them). Intense solar heating keeps a very
    > shallow layer of air warm, even though the heated air is rising and
    > dispersing while cooler air is descending into contact with the land
    > where it is rapidly heated its turn.
    > Trevor Kenchington
    > --
    > Trevor J. Kenchington PhD                         Gadus@iStar.ca
    > Gadus Associates,                                 Office(902) 889-9250
    > R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    > Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    >                     Science Serving the Fisheries
    >                      http://home.istar.ca/~gadus

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