"Maybe the liquid has to come from an external source.  I was wondering does 
anyone know if snow changes Ranma into a girl?"

According to the [non-canon?] anime,  snow does not change Ranma.  He has 
bee hit in the face by a couple well aimed snowballs,  if I remember 
correctly.  Maybe it has something to do with the molecular properties of 
WAter,  H20.  Snow and ice has different molecular properties,  as does 
sweat, soda pop, etc...