{"id":2021,"date":"2011-06-22T20:37:02","date_gmt":"2011-06-23T03:37:02","guid":{"rendered":"http:\/\/blog.kootenay-lake.ca\/?p=2021"},"modified":"2011-06-24T01:29:10","modified_gmt":"2011-06-24T08:29:10","slug":"halo-substitution","status":"publish","type":"post","link":"https:\/\/blog.kootenay-lake.ca\/?p=2021","title":{"rendered":"Halo substitution"},"content":{"rendered":"

Today the cirrus contained columnar ice crystals, rather than plates. This meant no circumhorizontal arc, but other haloes compensated.<\/p>\n

At noon, for a few weeks around the summer solstice I watch for the circumhorizontal arc. This is a really very colourful arc lying parallel to and just above the horizon. Actually, it is not all that rare when the Sun is high enough in the sky for it to form. The ideal solar elevation is 68\u00b0.\u00a0At Nelson, the Sun reaches a maximum elevation of 64\u00b0 at solar noon (about 12:50DST) on the summer solstice (June 21). That\u2019s close, so for maybe an hour a day, for a week a year, the arc might be able to be seen from around here.<\/p>\n

Today was sunny and there was cirrus\u2014almost optimal. The only other needed ingredient in the recipe was that the cirrus had to contain the requisite prisms: plate\u2013like ice crystals. Now, most ice crystals in the atmosphere come in two basic forms (with lots of variations), hexagonal columns and hexagonal plates. Think of the columns as wooden pencils scattered randomly on a table; think of the plates as hexagonal dinner plates, again scattered over the table. The formation of the circumhorizontal arc requires the plates; today the cirrus delivered columns.<\/p>\n

However, the columnar ice crystals did give a nice display\u2014albeit not the one I sought. The picture below shows three haloes: The inner circle around the Sun (which looks too large owing to overexposure) is the 22\u00b0 halo. Surrounding it is an elliptical shape called the circumscribed halo; it is coincident with the 22\u00b0 halo at the top (upper part of the picture) and bottom (out of the picture), but shows a small separation on the left and right. At this solar elevation these haloes are caused by the columns. The centre of each of these haloes is the Sun.<\/p>\n

The Kootenay Lake website offers other pictures and more discussion of these haloes<\/a>. To get a sense of just how lovely the circumhorizontal arc can be, see the posting about its low\u2013sun brother, the circumzenithal arc<\/a>.<\/p>\n

A third halo appears, but is a bit subtle: the parhelic circle. This is a white arc that circles the sky at the elevation of the Sun. In this picture it is seen passing through the sun, and cutting across the other haloes at about 2 and 10 o’clock. The centre of this halo is the zenith, which is locate just above the 22\u00b0 halo.<\/p>\n

What can I say: it was a nice display, even if it wasn\u2019t what I hoped to see.<\/p>\n

\"\"<\/p>\n","protected":false},"excerpt":{"rendered":"

Today the cirrus contained columnar ice crystals, rather than plates. This meant no circumhorizontal arc, but other haloes compensated. At noon, for a few weeks around the summer solstice I watch for the circumhorizontal arc. This is a really very … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[12],"tags":[],"class_list":["post-2021","post","type-post","status-publish","format-standard","hentry","category-weather"],"_links":{"self":[{"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/posts\/2021","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2021"}],"version-history":[{"count":13,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/posts\/2021\/revisions"}],"predecessor-version":[{"id":2049,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=\/wp\/v2\/posts\/2021\/revisions\/2049"}],"wp:attachment":[{"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2021"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2021"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.kootenay-lake.ca\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2021"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}