Why do some planes leave long trails, but others don’t?

Why do some planes leave long trails, but others don’t?

The first thing it’s not smoke. Those long-lasting white traces in the sky are condensation trails (“contrails”) from all sorts of aircraft engines leaving water vapor in the air

Planes flying through the dry air (eg: Outside a cloud) DO NOT generally form contrails, but those flying through the wet air (eg: Inside a cloud) DO form contrails.