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    Re: Refraction. was: Bubble Horizon Altitude Corrections
    From: Richard B. Langley
    Date: 2004 Jul 5, 23:25 -0300

    On Mon, 5 Jul 2004, George Huxtable wrote:
    >Fred Hebard has an inquiring mind, and a tendency to ask interesting
    >questions. The one copied below came to me off-list
    >A private question, which you could make public if you wish.  Are there
    >ever events in the atmosphere where astronomical or distant earthly
    >objects will appear lower than they really are, rather than higher, ie.
    >events where the effective index of refraction is of opposite sign to
    >that usually encountered?  By effective index of refraction, I am
    >trying to indicate the total refraction between the object and the
    >observer rather than a local refraction.
    >Atmospherics is not my specialty, though I'm as willing to pontificate
    >about it as the next man. I'm posting Fred's question to the list, in the
    >hope than someone will pick it up who knows more than I do.
    >The quick answer is: I don't know, but think it's very unlikely.
    * * *
    Just before reading that message, I read a message from another group to which
    I subscribe, the Sundial List. It's not related to an astronomical observation
    but is still interesting. I quote that message verbatim.
    -- Richard Langley
    Hello sundial lovers,
    this is a bit off topic, but the sort of thing some of you might enjoy.
    Working at the airport has its advantages. Just today, I saw the Queen's airplane arrive
    (security assures me the woman stepping out was NOT the Queen, nor was the other lady the
    Minister whom I - and the policemen - seemed to recognize). Another tourist attraction was
    the test flight with the 1948 vintage Constellation.
    What amused me - and the technicians who had the time to look - most was this.
    Although all runways have radio beacons, there are PAPI lights as well.
    While working on the antenna array of the localizer on a particular runway, traffic was
    starting in the opposite direction on that runway, i.e. away from us.
    Occasionaly, a heavy would come in to land.  _That_ direction has no localizer, so the
    PAPI (Precision approach path indicator) lights were on, at full brilliance to fight the
    We could just see them over the slight bluff, but whenever a plane started, the lights
    would disappear behind the bluff. Stepping on a car tyre brought the lights back; the
    difference was about half a meter!
    First I thought the hot air from the jets caused this, but later I realized that hot air
    would help makes the lights visible (go "up"), not invisible. It was the other way 'round.
    The hot air over the bluff was disturbed by starting aircraft, which would mix in vast
    amounts of cooler air from a few meters up, making the light rays go straighter.
    Then after a few minutes, the air would be hot enough again to bend the light from the
    PAPI lights over the bluff.
    Everyone I told found it hard to believe that this would be so strong an effect over just
    a few hundred meters. Most took pictures when they had looked and seen it for themselves!
     Richard B. Langley                            E-mail: lang@unb.ca
     Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
     Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
     University of New Brunswick                   Fax:      +1 506 453-4943
     Fredericton, N.B., Canada  E3B 5A3
         Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/

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