On Mon, 5 Dec 2016 19:16:07 +0000
Mike Causer wrote:

This evening around sunset there was some odd-looking cirrus. The
oddity was that the curvature seemed to be in the vertical plane, not
horizontal. I observed it over a period of 30 minutes from positions
about 1km apart (home to get camera and out again to find a fairly
clear horizon) and that impression persisted. Location East
Cambridgeshire.

https://www.flickr.com/photos/54731504@N04/31405011106

That cirrus is perfectly normal; the curvature always occurs in the
vertical due to the ice crystals falling through layers of air having
differing wind velocities.

--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
Web-site: http://www.scarlet-jade.com/
There are more fools than knaves in the world, else the knaves would
not have enough to live upon. [Samuel Butler]