Circumzenithal Arc and Cloud Type

Thin cirrus generally gives the best halo effects but the cloud appearance isn't always a good guide to whether halo effects will be visible.

This was quite a bright circumzenithal arc, considering the thickness of the cloud, although the colours weren't as strong in the thicker parts of the cloud.

Zooming in a bit as the cloud drifted off. The colours were much more saturated when the arc was in the thinner parts of the cloud.

Although numerous sundogs suggested there might be circumzenithal arcs they were mostly faint or obscured by cloud. This one would easily have been missed by a casual observer.

Fortunately a large gap in low level cloud coincided with a very smooth and colourful arc. The sun altitude was 15 degrees, lower than the optimum altitude but it still produced a bright arc. This one was obvious to anyone who looked up.

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