What does Capulet mean: "When the sun sets, the air doth drizzle dew; But for the sunset of my brother's son it rains down right"?
In Act 3, Scene 5 of Romeo and Juliet, to what is Capulet comparing Juliet in this metaphor?
In Romeo and Juliet, it is ironic Lord Capulet, who himself has exemplified irrational and impetuous behavior in Act One when, seeing Lord Montague, he shouts for his "long sword" with which to slay his enemy, now in Act Three issues a speech of wisdom as he disparages the excessive emotion show upon the death of Tybalt:
When the sun sets, the earth doth drizzle dew [some cry];/But for the sunset [death] of my brother's son/It rains downright [it seems that everyone is crying]....How now! a conduit [fountain], girl? what, still in tears? (III,v,132)
Since the first two lines are a rather philosophical reflection upon life in general, any metaphor comparing Juliet does not come until the line after what has been quoted: "How now! a conduit...." Capulet believes that his daughter's tears, to which he compares a sea, are creating such an emotional storm ("Thy tempest-tossed body" l.140) that a calm needs to settle upon her, even though he was angered at Lady Capulet for her efforts to calm him in Act One. Thus, he asks his wife if she has told Juliet about her marriage proposal, believing this occasion will be the "sudden calm" upon "the bark [ship]" (III,v,136) that is Juliet.
It's not clear to me that Capulet is really referring to Juliet in this metaphor. I think that the lines you quote aren't really about her, but are just general observations.
If he is referring to Juliet, he is comparing her to the weather, to the natural processes that make rain. He is saying that natural sunset causes dew, but that the sunset (death) of his nephew (Tybalt) causes real rain (heavy tears). So if he's referring to Juliet, she would be the clouds that the rain falls out of. (He thiks she's crying because Tybalt is dead.)
Just after this quote, Capulet goes on to compare Juliet to a bunch of things -- a fountain (conduit), a ship, the ocean, the wind.