Contrails in autumn

[tracymort]

I've noticed the past couple of weeks there are far more planes, or rather far more contrails alerting me to the aeroplanes in the sky. Is there an explanation to why I'd see this at this time of year?

[excelsior]

i'm guessing it's due to the fact that the air is colder now - but i'm not an expert

[Teddio]

Aeroplane jet engines burn kerosene which produces carbon dioxide, water vapour and a tiny amount of microscopic carbon particles. Above about 30000 feet the air is very cold and often supersaturated with water vapour. In these conditions the tiny particles of carbon act as nucleation centres around which the vapour desublimates into ice crystals. The ice crystals themselves act as nucleation centres and they grow forming artificial cirrus clouds, contrails. These ice crystals usually sublime back to the vapour phase and the contrail disappears.
We've been having a lot of anticyclonic weather lately with clear skies. I don't know whether there's a seasonal factor, contrails are very common at the height of summer, they occur in the stratosphere, well above "normal weather"

[SteveD]

I think that the most I have seen at any one time is 10. it just goes to show how many aircraft there are up there although it must depend where you are in relation to the air corridors overhead. I am just south of Brussels which must be like Spaghetti Junction!).