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This morning’s haloes, forming over the Lake, might have been expected. After all, a storm is moving towards us and is expected to bring rain overnight\u00a0and tomorrow. Often such events\u00a0are presaged by a canopy of cirrus, which is just the thing to give haloes. Yet the sequence of a halo preceding a storm, while common on the Coast, is not all that common around here.<\/p>\n
The appearance of these circles, arcs and spots in the sky requires the ice crystals to have simple shapes. The stellar crystals, beloved of shop decorators around Christmas, just don’t do it. Rather, what is needed is either a hexagonal column (somewhat like a wooden pencil with a hexagonal cross section), or a thin slice through that (somewhat like a hexagonal dinner plate). The growth of these shapes results from the small supersaturations that are characteristic of\u00a0the slowly ascending air found as a storm approaches from the ocean. The more chaotic air motion over the mountains seems to favour higher supersaturations and the more complex crystal forms.\u00a0<\/p>\n
Sometimes we do see the simple crystal forms in the sky. What was interesting about this morning’s haloes was that they only resulted from\u00a0columnar crystals, not plates. Which\u00a0form appears depends not only on the humidity, but also the temperature in the cloud.\u00a0<\/p>\n
In the scene, below, the 22\u00b0 halo is the smaller one centred on\u00a0the Sun. It is formed by quite small crystals that assume a wide range of orientations as a result of the Brownian motion in the air. Sunlight passing through the 60\u00b0 prisms presented by alternative crystal sides is deviated by a minimum angle of about 22\u00b0 to give the halo. As the columnar crystals grow larger they become horizontally oriented by aerodynamic forces during their descent. These crystals give the upper tangential arc that appears as the bird wings touching\u00a0the top of the 22\u00b0 halo.<\/p>\n
At about twice the distance from the Sun as the 22\u00b0 halo is another one. This is explained by sunlight refracting through the 90\u00b0 prism ends of columnar crystals. If these crystals were small and assumed a wide range of orientations, they\u00a0would produce the 46\u00b0 halo. It is more likely that what we are seeing is the result of larger columnar crystals producing the\u00a0supralateral tangential arc.\u00a0<\/p>\n
Be that as it may, the features that would be caused by hexagonal plate crystals — sundogs, circumzenithal arc — are missing. To see a posting with some of these other features, visit celestial splendour<\/a>.<\/p>\n This morning’s haloes, forming over the Lake, might have been expected. After all, a storm is moving towards us and is expected to bring rain overnight\u00a0and tomorrow. Often such events\u00a0are presaged by a canopy of cirrus, which is just … Continue reading ![\"\"](\"https:\/\/blog.kootenay-lake.ca\/wp-content\/uploads\/2015\/10\/halo151018s.jpg\")
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