As stated the SCWD needs a nuclei round which to freeze. An a/c performs this job exceptionally well, so getting iced up.
Similar thing ocurs with what could be called 'super cooled water vapour', inasmuch that water vapour needs a nuclei around which it can condense to form a cloud. Usully there are so many particles in the air (smoke, dust etc)that this happens with no problems. However in very clean,cold enviromnents, such as Antartica, there are sometimes not enough particles to allow condensation to occur. When you get out of your hut on a crystal clear morning and start up the generator, the exhaust smoke allows the condensation to occur, and cloud quickly forms.