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    Re: Refraction near the Horizon ? Observation vs. Calculation
    From: Brad Morris
    Date: 2013 Apr 6, 17:30 -0700

    Hi Marcel

    Thank you for your attention to this discussion. I was able to follow your procedure and indeed, have created a graph of the dip when affected by pressure and temperature.

    What temperature and pressure? Why the highest values recorded on earth, as recognized in Wikipedia. The highest pressure was 32inHg=1083mb. The lowest (non-tornado) pressure was 26inHg=880mb. The highest temp was in Death Valley 56.7C=329.8K. The lowest temperature was in Antarctica -89.2C = 183.9K. You may find a graph of this, along with the nominal P&T, up to 304 meters.

    I then went ahead and pulled out the values for 31.312 meters (103 feet, corresponding to the bridge of Jeremy's merchant vessel).
    Pressure Temperature Dip(P,T)
    1010mb 283.15k 9'49.9"
    880mb 283.15k 9'57.0"
    1083mb 283.15k 9'45.9"
    1010mb 183.9k 10'9.3"
    1010mb 329.8k 9'40.8"
    You may wish to confirm these values.

    These values are remarkably different from the Nautical Almanac. Using the A4 Altitude Correction Tables", we find that our temperature and pressures are well beyond those charted, but we can just take the values shown to see what the adjustment should be. I will use the "App Alt" to be 0deg0min.
    The correction for 1010mb -30C is an additional -7.3 minutes.
    The correction for 1010mb +35C is an additional +6.9 minutes
    The correction for 970mb 10C is an additional +1.0 minutes
    The correction for 1050mb 10C is an additional -1.1 minutes

    It is possible that I did not perform your procedure correctly. Its also possible that the Nautical Almanac is wrong or that I used it improperly. Failing that, you must try to understand why the variation for pressure and temperature as given by the NA differs so wildly from your results.

    Regards
    Brad

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