Photo of Bluebell Woods in Sussex Photo of Bluebell Woods in Sussex

Circumzenithal Arcs

My thanks to Simon Cadman for the following generous email and some great images of some sun dogs and the more rarely-spotted, ‘upside-down rainbow’, also known as a ‘circumzenithal arc’ or ‘Bravais’ arc’.

Circumzenithal arcs and sun dogs form when sunlight refracts through ice crystals, not rain as with rainbows. This usually indicates cirrus or cirrostratus clouds. Both these clouds can presage a front and deteriorating weather.

Hi Tristan

I read your book on holiday it’s changed my life forever.

I saw a reverse rainbow and some sun dogs recently the rainbow was between me and the sun and was a happy face in the sky rather than sad face rainbow, its angle was about 80 to 85 degrees vertical towards sun, I think there was a temp inversion. It was early evening.

To be honest I wouldn’t have seen them without having read your books , I now see more than I ever could have imagined, that was right in front of me.


And my thanks to Michalis Kouloumis for sending over this example of a circumzenithal arc too:

My thanks to Catherine McKee for this excellent example of a circumzenithal arc at the top of a 22 degree halo:

The upper tangent arc is the smile at the top of the photo above.

It has formed at the top of the large 22 degree halo.

Both are a sign of cirrus clouds and therefore moisture in the form of ice in the upper troposphere.

On their own these arcs do not signal bad weather approaching, but if followed by cirrostratus, then a warm front and rain are probably on the way