Welsh Rarity
Jon Trew caught this circumhorizon arc fragment at Barry, Glamorgan, South Wales on 21st June.  Top picture Nikon, lower iPhone.

What is so very rare and unusual about it? Read below.

Image ©Jon Trew, shown with permission.


About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed
This halo needs the sun high, above 58°.

That only happens near noon for a few days around the summer solstice In the UK and similar European latitudes.

Then the halo hugs the horizon and we must contend too with cloudy skies! As I write this at midday I keep glancing through the window to check the south for just a glimpse of just a tiny fragment of one. The sun shines strong and there is some cirrus but the horizon is congested with woolly cumulus. Wait, now the sun has faded - maybe tomorrow, maybe at next year’s solstice or the year after!   They are a very rare and cherished sight here.

American and more southerly cousins be pleased for with your higher sun and clearer skies it is a much more frequent spectacle. With careful sky checking you might see it several times each year.

When complete the halo is huge. It is two outstretched hand spans below the sun and sweeps partly around the horizon.

Horizontally aligned hexagonal plate ice crystals in high and freezing cirrus are responsible. Light enters a vertical side face and exits the base. The refraction through the equivalent of a 90° gives the widely separated and pure colours.

The sun was almost 60° high when Jon Trew saw his fragment. The ice crystals appear to be in a band of cirrus that has developed and spread from an airplane contrail.