How come contrails only cover the sky some days when planes fly every day?

Question

How come I never remember them covering they sky as a kid?

Answer 1

It depends on atmospheric condition for them to show.

Answer 2

this question is asked in some form or another quite frequently. And the answer is that it depends on the atmospheric conditions at cruising altitude. water vapor freezes (crystalises) at a certain temperature, and if the temperature at altitude is below what is required for water vapor to crystalise, then no contrail will appear.