Is it possible for clouds to form in a big enough cave?

Fog could certainly form but for the normal cumulus or cumulonimbus clouds you would need convective currents to drive the moist air upwards. For fairly moist air an ascent of a few thousand feet would suffice. For cumulonimbus clouds you would need 10's of thousands of feet. But it is your story, and you could include strong magnetic fields to induce some electrical charges as well.