On the 17th August, not long before sunset, part of what seemed like a
large horizontal rainbow appeared beneath the clouds.

I took a photo of it, though to be honest the picture doesn't capture it
very well.

http://www.btinternet.com/~nickgardn...talrainbow.jpg

There was only part of the arc, but I am assuming that it would have
made a complete circle overhead given a cloud-free sky towards the
western horizon.

With the red of the setting sun, the colours of the rainbow were
obscured somewhat, but even so, you could make out the blue colours
quite well.

I've seen circumhorizontal arcs before but this was different in that it
was much larger and appeared directly beneath the cloud. Also the sun
needs to be high in the sky, whereas here it was just above the horizon.

I suppose it was caused by rain falling from the cloud, being lit up
from below due to the low sun, and the rain then evaporating quickly as
it fell towards the ground.
____________________________
Nick G
Otter Valley, Devon
83 m amsl
http://www.ottervalley.co.uk