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The circumzenithal arc is only visible when the solar altitude is below 32 degrees.
Near the upper limit the arc is just a small patch close to the zenith, when the sun is near the horizon it is spread out over a large arc.
It is most prominent when the solar altitude is around 22 degrees.
For London circumzenithal arcs can be seen any time of day from mid October through February, if the conditions are right.
In this timetable the altitude range has been arbitrarily split into 5 degree sections to demonstrate how masks can be used to highlight altitude specific phenomena. Light blue is when the sun is too high, the darker blues are when the sun is below the horizon. See below for details of how to reproduce this diagram. |
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When GraphDark is installed the default is to show midnight across the middle of the timetable. The first step is to set up the timetable to show midday instead, and also to increase the number of days per screen. Use Edit, Preferences, Timetable Display, Go to the Times frame and check the Day Mode box. Go to the Day Width frame and select Very thin Return to the graph and choose your site from the Where box. From the What box select Sun then click on the M button, just to the right, to select an altitude mask. Choose Circumzenithal Arc from the list and click OK, the graph will be drawn immediately. To change the colours used for each altitude band simply go to the mask selection window. Double click on the band and choose a new colour. Circumzenithal Arc was chosen as an example to be shipped with the application, but masks can be set up for any altitude specific phenomena. See Altitude Masks for more examples. Return to Glows Bows and Haloes |
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