"It is too cloudy."
Translation:Mae hi'n rhy gymylog.
Rhy is one of the few adjectives that come before a noun and causes a soft mutation, as does 'hen' (old) which is another of the adjectives that come before a noun.
The letter rh in rhy resists the usual soft mutation following this usage of yn. This kind of 'contact mutation' is in any case not passed on to subsequent words.
Here, rhy is an adverb ('too') modifying the adjective cymylog, and it is rhy itself that causes the soft mutation of c to g.