Callistemon21
I've heard of cloud seeding but I thought vapour trails were just that - vapour trails left by high-flying planes.
It's sunny but hazy today so I can only see two.
I agree, I live under the Heathrow flight path and see so many of these vapour trails, including darker ones where fuel is dumped - though we are told this doesn't happen we can actually smell aviation fuel some days.
Some days there are trails crossing everywhere, but if you watch them you will see that they are blown in variable directions/speeds with the winds at different levels in the sky.
It isn't unusual to be able to see a number of planes going in several directions at the same time, taking off, landing or simply flying over at a higher level, and these trails are blown all over the place.
In some calm weather conditions they can last for hours, often becoming wider and paler.