Homogeneous Nucleation of a fog, anybody?

Hello all,

I was asked (that looks like) a simple question:
Why is homogeneous nucleation of fog a dificult process?

Searched the net for the answer, and found no simple explanation of the
subject.
Apparently everything that goes from isotropy to anisotropy requires a
bunch of energy to start a germ formation, but how to explain it more
thermodynamically...

.