The message that Hamlet is trying to convey to his old 'friends' and the audience is C: Hamlet is saying that his madness changes like the weather, and that he is only mad some of the time.
This is clear from the phrase "when the wind is southerly I know a hawk from a handsaw" after having mentioned other cardinal directions before and linking them to him being mad. This distinction seems to suggests that the weather affects his madness (or more logically, that his madness changes like the weather). Therefore, sometimes he sees clearly (and can know a hawk from a handsaw) and other times he is completely mad with revenge.
