In message , Anita Evans
writes
BTW - yesterday evening the cloud seemed to be arranged in approx east
west stripes, and some of the contrails were in what I can only
describe as a helix pattern.

Now I think about it, I can probably answer my own question as the
contrails were at right angles to the cloud stripes, so I assume
sideways air movement combined with the momentum of the trail would give
a helix effect.
--
Anita Evans
North Cumbria
(anita[at]ra.evans.clara.co.uk to reply by e-mail)