Contrails are effectively artificially made streaks of cirrus clouds, formed in cold, moist air, high in the atmosphere generally.
They are CONdensation TRAILS formed by aircraft wings or exhausts.
Some people think they scar the sky and serve as a constant, visible reminder of our energy-wasting society and our need to reduce our.... carbon footprint... (ssshhhh).
Research is constantly being carried out, it would seem, as to whether or not contrails change our weather. They can certainly block out the sun sometimes, on a 'clear' morning in parts of the UK.
Living quite close to London, and Heathrow in particular, contrails are often part of our clear skies here.
I took a few shots this morning, some of which can be found in this post and the posts above...
This first one is quite obviously a sign that Scotland WILL beat Argentina in the Rugby World Cup quarter finals at the weekend...