Circumhorizon Arc Visibility

The possible light paths through almost horizontal plate crystals sets limits on when the circumhorizon arc can be seen. Because it is only formed when the sun is above 58 degrees it isn't seen very often from much of Europe, and not at all for the more northerly regions. The sun really needs to be a few degrees higher for a good view as the simulations below show. Things improve up to around 75 degrees solar altitude and then the intensity drops off again. By the time the sun gets to 85 degrees very little light gets through the vertical side faces of the crystals so little or no arc is formed.

Each slice is for a different solar altitude and shows the sky from the horizon up to about 70 degrees. The sun shows as a small circle in the first three slices. The solar altitude in degrees is along the bottom.

Montage prepared using screenshots from HaloSim

The dependence on latitude affects the amount of time it is possible to see a circumhorizon arc. The diagrams below show how this varies by time of day throughout the year for a number of latitudes. For lower and tropical latitudes the cutoff when the sun is near the zenith becomes more pronounced. This results in fewer hours than might be expected, but still far more opportunities than further from the equator. In the southern hemisphere most of the population lives closer to the equator so this effect is more significant.

The timetables below were prepared using GraphDark and show all the hours of daylight through the year for different locations. Calendar date increases from left to right while each day is represented by a vertical line with morning at the bottom and evening at the top. Local noon runs across the middle with the light blue area representing the hours of daylight. The colour gives an indication of how suitable the solar altitude is, with orange and red being more favourable. Yellow is used when the sun is between 58 and 62 degrees, orange 63 to 67, red 68 to 72, orange 73 to 77, yellow 78 to 82, 83 and above is light blue to indicate no arc.
Click on the small timetables for more detailed versions.

London, 52 degrees north.

The sun reaches 62 degrees at the summer solstice so a circumhorizon arc can form. In theory it can be seen for an hour or two each day from mid-May to mid-July but it is likely to be very faint near the limits.

Click the timetable for a more detailed version which shows the actual sightings from previous years as red dots.

Madrid, 40 degrees north.

The sun gets to the altitudes where the arc is strongest, the regions in orange and red. The season is also much longer lasting from April through August.

Tenerife, 28 degrees north.

This is now in the region where the upper limit on visibility starts to have an effect. Local noon at the summer solstice is one time you won't get to see the arc, in most of Europe it is the best time.

Dakar, 15 degrees north.

Just inside the tropics the sun passes directly overhead on two dates each year. In theory the arc could be seen at noon on the solstice again.

Nairobi, 1 degree south.

The effect of the sun's north-south movement is even more pronounced.

There are now hundreds of hours a year when the arc could be visible, but of course it all depends on having the right kind of crystals in the right place at the right time.

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