Abstract
The results of a quantitative treatment of the visibility of the circumzenithal arc as a function of solar elevation are compared with records of the arc’s occurrence in France and the Netherlands, 1894–1931. Both calculated and observed frequency distributions peak at a solar elevation of 22° (minimum deviation), and the width of the observed frequency distribution can be closely matched with the assumption that plate ice crystals in quiet air undergo oscillations of around 1° from equilibrium. This result agrees with other estimates based on studies of the subsun and parhelic circle. The circumzenithal arc is shown to be vertically polarized, with Ivert/Ihor ≈ 1.4 in the solar vertical near minimum deviation.
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