Sometimes they're there, and sometimes not. Can you explain that to me?

I’m having trouble looking this up on my own, for some reason, and admittedly, Science was one of my poorest subjects, so that’s why I am asking for you to explain something to me.

Those white, sometimes billowy lines seen behind jet aircraft are called CONTRAILS, but they aren’t visible every day, even though jet aircraft fly above my home, every day. So my question is, why aren’t they always visible to me? Why is it that some days, they are clearly visible (and incredibly long) and other times, they’re not visible at all?

I’m assuming (based on my limited science knowledge) that relative humidity, altitude, light refraction and condensation all play a role in the “visibility factor,” of the contrails.

So, can you please explain how I can sometimes see them, and on other days, they aren’t visible at all? Also, sometimes they are very long (and stay visible for what seems like hours) and other times, they are very short, and disappear quickly.

Thanks for contributing to my education, should you choose to post an answer here, and don’t hesitate to “talk down” to me, on this particular subject, because I freely admit my ignorance about it.