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    Effect of Relative Humidity on Index of Refraction
    From: Fred Hebard
    Date: 2003 May 3, 12:24 -0400

    I like this guy Snell, whose law says that we don't have to worry about
    the refractive index above our elevation!
    
    I evaluated the formula in the Smithsonian Meteorological Tables on
    page 389, to examine the effect of relative humidity on the index of
    refraction.  By my calculation, it doesn't show up until the fifth
    place in the index of refraction.  So, for the purposes of nautical
    astronomy, it needn't be considered.  (And one would expect this since
    the Nautical Almanac doesn't consider it)!
    
    Here are the results in tabular form.  p is the atmospheric pressure in
    millimeters of mercury.  Sat vp is the saturation vapor pressure of
    water in _inches_ of mercury.  t is the temperature in degrees Celcius.
      lamda is the wavelength of light in micrometers.  rh is the percent
    relative humidity.  f is the vapor pressure of water at that humidity.
    term1 is the first term of the Smithsonian formula, which accounts for
    atmospheric pressure.  term2 is the second term, which accounts for
    water vapor pressure.  n is the index of refraction.  I may not have
    converted properly between inches of mercury for water at saturation
    vapor pressure for that temperature (Sat vp) and the partial pressure
    from water in millimeter (f), but, if my evaluation of the formula is
    correct, an improper conversion would not be significant.
    
    You can see that term1, the effect of atmospheric pressure, dominates
    the numbers, and that the index of refraction is not strongly affected
    by humidity.
    
      p    Sat vp     t  lamda     rh   f         term1      term2      n
    
    760   0.04477    0  0.5461     0   0         297.7584   0
    1.0002977584
    760   0.04477    0  0.5461   100   0.113716  297.7584   0.006621
    1.0002977518
    760   0.05646    5  0.5461     0   0         292.3885   0
    1.0002923885
    760   0.05646    5  0.5461   100   0.143408  292.3885   0.0082
    1.0002923803
    760   0.0708    10  0.5461     0   0         287.2082   0
    1.0002872082
    760   0.0708    10  0.5461   100   0.179832  287.2082   0.010101
    1.0002871981
    760   0.08832   15  0.5461     0   0         282.2077   0
    1.0002822077
    760   0.08832   15  0.5461   100   0.224333  282.2077   0.012382
    1.0002821953
    760   0.1096    20  0.5461     0   0         277.3778   0
    1.0002773778
    760   0.1096    20  0.5461   100   0.278384  277.3778   0.015103
    1.0002773627
    760   0.13534   25  0.5461     0   0         272.7099   0
    1.0002727099
    760   0.13534   25  0.5461   100   0.343764  272.7099   0.018337
    1.0002726915
    760   0.16631   30  0.5461     0   0         268.1959   0
    1.0002681959
    760   0.16631   30  0.5461   100   0.422427  268.1959   0.022162
    1.0002681738
    760   0.20342   35  0.5461     0   0         263.8285   0
    1.0002638285
    760   0.20342   35  0.5461   100   0.516687  263.8285   0.026667
    1.0002638018
    760   0.24767   40  0.5461     0   0         259.6005   0
    1.0002596005
    760   0.24767   40  0.5461   100   0.629082  259.6005   0.031949
    1.0002595685
    
    
    

       
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