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    Re: Refraction. was: Bubble Horizon Altitude Corrections
    From: Trevor Kenchington
    Date: 2004 Jul 9, 19:58 +0000

    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{at}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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