Last night we had  a halo around a three quarter waxing gibbous moon and a sight more beautiful than my photo can possibly describe. It appeared to be an enormous hole in a thin layer of alto cirrus cloud, but could the moon generate enough radiation to evaporate this high altitude moisture? We needed the advice of an expert. So I e-mailed Tristan Gooley (see previous post) this morning and asked for advice…

Hi Simon,
Hard to be certain from one photo, but it looks like a similar phenomenon to a ‘moonbow’. Rainbows, fogbows and moonbows are all arcs, often coloured, caused by reflection and refraction of the light from the sun or moon hitting water particles in the atmosphere.
In this case the moisture may be a thin layer of cirrus cloud. The Arctic Innuit and Pacific islanders have been known to use this effect to forecast bad weather, in western terminology, they can herald an approaching front.

I may be completely wrong though!

best, T

Ah, an approaching front. Let’s check the weather forecast…

there it is.  We are where I have put a red dot. The front is moving to the north of us on a warm south westerly wind. This is often why the east coast of Spain is dry, weather systems seem to move to the north of us held at bay by some Mediterranean high pressure.