Malden, 2nd June 1991

Photo by Peter Paul Hattinga Verschure *

In case of a relatively homogeneous Cirrostratus cloud one might expect to approach the diamond dust conditions, where intensity differences of different halos tell something about the ice crystals and are not just signs of cloud inhomogeneities.

In Peter Paul Hattinga Verschure's wonderful midsummer photograph the cloud causing the halos seems smooth. Therefore it seems possible to try to find out, using simulation program, if the aspect ratio of the column crystals can be deduced from the relative intensities of the parhelic circle and Wegener anthelic arcs. It will also be interesting to see whether the brightening at the anthelic point can be reproduced with the simulation program.

Here the assumption, that plate crystals are not involved in the creation of the halos (namely parhelic circle), has been made, and only one column population was used to simulate the display. With short prisms the number of Wegener anthelic arc rays is close to the number of rays of parhelic circle in the simulation. Elongating the prism increases the gap for the benefit of parhelic circle, but the difference never grows remarkably.

Increasing tilt is out of question, since it deteriorates both halo forms quickly. The photo shows well defined, narrow halos, which is a definite sign of well oriented crystals. If the hexagonal profile is allowed to vary towards more triangular shape, the parhelic circle is the sole victim of this and is therefore out of question. It seems quite difficult to produce correct relative intensities between parhelic circle and Wegener anthelic arcs with just one column population. Perhaps plate crystals contributed for the intensity of the parhelic circle after all. A reasonably good match with the photograph was obtained with a column crystal having aspect ratio of 2 and normally distributed tilt with std of 0.3 degrees.

The anthelic brightening really comes up in the simulation. See how the different parameters affect its appearence. The parameter file of the simulation above can be downloaded HERE.


Back to Examples main page

[*] Halo photograph © Peter Paul Hattinga Verschure, shown with permission. More Peter Paul's images can be found from his site.