High-bypass Turbofan Engines cannot produce trails of any kind
We will be writing this page up more fully soon, but essentially modern jet engines are incapable of producing any trail at all, even in ideal conditions, due to the fact that they do not produce enough water vapour, so even in conditions ideal for natural contrail formations, which are at temperatures lower than -57 Celsius, which equates to an average of around 35,000ft +, no visible trail can form and certainly not a persistent trail of any kind.
NB All larger UK and European passenger airlines use either Boeing and Airbus aircraft, both of which use only High-bypass Turbofan Engines. This means every trail we see from any large passenger jet is being produced by aerosols released during flight. There is no such thing as a natural contrail anymore, except by much older jets in extreme conditions, but most of these older jets have been rtured now, so a natural contrail should be a very rare sight anywhere, let alone in Europe and yet we see images such as this on a daily basis in the UK
This is why we know that aircraft which travel over London at altitudes as low as 16,000ft on summers days are releasing something into our atmosphere during flight.
This is illegal and yet the CAA do not seem to be interested in this problem.