Muktzeh - Catching rainwater

Rabbi Moishe Brandeis sent the following question to Rav Ostroff:

While in Manchester on shabbos visiting my parents, my father raised the following point.

What is the difference between air conditioning water and rainwater, where we find that you may move a kli to catch the leak. Rainwater, says my father (the chemical engineer), has exactly the same process as a dripping air conditioner unit.

I hazarded a guess at bidei shomayim and bidei odom, but am not sure. Please will you enlighten me.

Rav Ostroff replied:

I'll try. The gemora says that rain is in the clouds and condensed air is not. It could well be that they are both the same, but as far as appearance goes, rain seems to come from somewhere, which is a reason to remove muktzeh, whereas condensed air is from vapor - as if from nowhere.

Shlomo Amar commented:

Maybe the rain was made before Shabbos in the clouds and therefore no issue of nolad, while the AC water was nolad on Shabbos.

Rav Ostroff replied:

Correct, the rain is in the clouds before Shabbos, whereas the AC water is "put together" on Shabbos.