An ice crystal halo display that contains several previously unknown halo phenomena was observed in Northern Chile. Analysis of computer simulations of the halos demonstrates that most of the new halo arcs in the display can be explained by the presence of airborne and preferentially oriented crystals of cubic ice. These observations therefore provide evidence of the existence of the cubic phase of ice in the Earth's atmosphere.

We develop a theory for a new effect on the parhelic circle. We show that there is a colored segment on the parhelic circle at a certain large azimuth that depends on solar elevation. The color segment appears at solar elevations below 32 degrees and is explained as a transition between total and partial internal reflection in the main ray paths of the parhelic circle in oriented ice crystals. Based on our simple refraction theory and computer simulations, we find that the color of the segment is mainly green and blue. The theory is tested with available photographs of the effect.

Many halo arcs are caused by pyramidal crystals that have {1 0 -1 1} crystal faces. We treat halo arcs arising from pyramidal crystals that fall in the air with their c axes vertically oriented. To our knowledge only 6 of the 12 possible halo phenomena that belong to this category have been dealt with in the literature. Surprisingly the yet undiscussed halos are predicted to be of comparable intensity with those already treated. They are produced by reflections from pyramidal crystal basal faces. A theoretical summary and computer simulations are presented of the mentioned 12 halo phenomena and of the individual arcs into which they break in the sky. We give an overview to the current level of documentation of these phenomena by listing the first published photographs of each phenomenon known to the authors.

Elliptical halos and Bottlinger's rings are simulated by the use of a Monte Carlo algorithm that includes multiple scattering. High multiple reflections are required to obtain a sufficient brightness of the elliptical halos. By introducing three populations of nearly horizontal, gyrating, and swinging crystals, we obtain good agreement for four photographs of displays with ringlike and disklike structures. Through model experiments on the aerodynamical behavior of crystals, we find a new interpretation of the three populations. In this view elliptical halos and Bottlinger's rings occur in the transition region of ice plates with broad branches and stellar crystals at temperatures near -15 °C and large supersaturations. This interpretation is supported by a review of 23 reports of elliptical halo phenomena by members of the Finnish Halo Observers Network.

The effect of wind speed and direction on the distribution of sewage-associated bacteria.