Why Are Contrails Visible Some Days but Not Others?

From where I am currently I have a great view of the sky in an area where a lot of high altitude traffic flies over, and notice that on some days aircraft contrails are clearly visible, while on others they are not at all?

It's cool to take a pair of binoculars and have a look at aircraft. But on the days where there are no contrails, they're not easy to spot.

Not that it's annoying, just interesting. I don't really mind when I can't see them, it's just a bonus on the days that contrails criss cross the sky. But I don't understand how can the weather be so different, that the same process reflects so differently in the sky.

Assume we have all clear blue skies, how can weather conditions be so different that on some days the water vapor condenses and is visible and on others it's not?