Another favourite puzzle for people is when contrails just disappear -a common site around London when the aircraft is at a marginal contrail level and suddenly enters an area of slightly warmer air and bingo no trail-
I livid in Bermuda for many years and this phenomena was very common- contrail over sea- disappears over island- re appears when over sea again
They are very capricious and complicated things as many here have suggested with a huge number of variables affecting their formation .
Given that government agencies tend to stuff up a lot of things they do the idea that any serious effort at chemical distribution via them is pretty remote- its like the use of poison gas in WW1-oh **** the winds changed and we are gassing our own side.
Pretty to look at tho and I miss them when in parts of the world that are 'off airways or after the volcanic ash problems
PB