Free Halo Simulation Program
© Jukka Ruoskanen 2010
Davos, 20th Dec 2005
Photo by Claudio Silberroth *
Most of the diamond dust halo displays that exhibit a Parry arc include a helic arc as well. In high clouds helic arc is rarely seen, but in diamond dust it is more easily encountered.
In Claudio Silberroth's photograph of a fine Davos display the upper suncave Parry arc seems strong enough for helic arc to be seen as well. In a diamond dust display of this magnitude one would also search for 46° Parry arcs, which might appear as brightenings on the supralateral arc. But they and the helic arc are missing from the photo.
In an attempt to simulate the display the populations of plate-, column- and random oriented crystals are quite easy to tune to satisfacorily approximate the halos of the photograph. The Parry population, on the other hand, requires some thinking. How can the helic arc and 46° Parry arcs be suppressed and yet allow the suncave Parry arc be formed?
One method is to modify the Parry oriented crystals profile towards tabular shape (see image below, crystal population 3). Such a step brings us immediately closer to goal, since the area of the faces 4,5,7 and 8 is suppressed. In the modified tabular crystal the probability for ray paths of helic arc as well as 46° Parry arcs to occur is lower than for the suncave Parry arc. In comparison with the case of a regular hexagon the tabular crystal virtually shuts helic arc and 46° Parry arcs down but while doing so does not markedly influence the suncave Parry arc. maybe this was the case in Davos display.
Try it yourself, and see if you can come up with a more plausible explanation. The parameter file of the simulation above can be downloaded HERE.
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