Why do planes leave a trail behind them?

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Tags

1. #leave
2. #reason
3. #which
4. #pressure
5. #contrails
6. #wakes
7. #engines
8. #sometimes
9. #humid
10. #mixes
11. #white
12. #their
13. #vapor
14. #altitude
15. #breath
16. #exhaust
17. #temperature
18. #trails
19. #lower
20. #atmosphere