Meteorologists would tell us that we see them sometimes and not others due to the varying temperature in the sky. Yet despite following these changes in temperature I can see no correlation.
They are put there intentionally. The big question in why?
This is the big question... WHY? @grateful1 living in Los Angeles really opened my eyes to the skies.... turning from blue skies at sunrise to white haze by lunchtime, without a single cloud in the sky. It's so obvious when they take the day off.... like Sundays, Christmas Day, or New Years day. I guess even Chemtrail Pilots need a break from spraying the world with blood and heavy metals! Bless em. http://www.onepagenews.com/tag/desiccated-human-red-blood-cells/
The trails I am looking at are commercial flights. I know this because I track them as they go.
So I always imagine they put the stuff in the fuel some days and not others and the pilots probably don't know the difference.
Video May or may not be from a real flight but, is interesting
https://m.youtube.com/watch?v=5Pn865TvpT8
I certainly wouldn't notice the difference if some days my car exhaust left a white line behind me everywhere I went. (insert sarcastic emoji here)