As I understand it, Nuvoletta ... disappeared in the end of the excerpt.
In Finnegans Wake, no one ever disappears, at least not for very long, because they always have to begin again!
While the song you are studying misleadingly ends (159.6-10)
Then Nuvoletta reflected for the last time in her little long life
and she made up all her myriads of drifting minds in one. She
cancelled all her engauzements. She climbed over the bannistars;
she gave a childy cloudy cry: Nuée! Nuée! A lightdress fluttered.
She was gone.
the very next lines in the book are (159.10-18)
And into the river that had been a stream ... there fell a tear,
a singult tear, the loveliest of all tears ...
for it was a leaptear.
But the river tripped on her by and by,
lapping as though her heart was brook:
Why, why, why! Weh, O weh!
I’se so silly to be flowing but I no canna stay!
What is the background of Nuvoletta (whom I suppose is a character in the novel) that makes such metaphor appropriate?
Nuvoletta is a form of Issy (I), the canonical girl child.
Issy has 28=(7x4) "nubied companions" (157.13). "Hence" Issy is 29, the leap-year child.
She has witnessed her twin brothers quarrelling across the banks of a river.
The text of Finnegans Wake is a river, and the mother of these
three children, Anna (ALP), is all rivers ("Missis-liffi" 159.12), in the never ending cycle river-sea-cloud-river.
As such, Anna is the renewer. Of course Anna was/is an Issy, and Issy must become an Anna.
Nuvoletta attempts to get her brothers' attention using "all the winsome wonsome ways" (157.31), but fails, ultimately because they are men ("There are menner" 158.5)
like their (step?) father HCE ("conclaved with Heliogobbleus and Commodus and Enobarbarus" 157.26).
Washerwomen (themselves forms of Anna) come and take the boys home, leaving Nuvoletta alone.
After leaping (suicide?), Nuvoletta becomes a tear, but falls into the river (her mother Anna). We know the tear is Novuletta (Issy),
since the tear is a "leaptear" and leaptear → leapyear.
Nuvoletta is reborn and flowing again, and no matter how she feels, she "canna" (Anna) remain at rest.
But what will happen if Anna stops flowing?
As I understand it, Nuvoletta is likened to wind, dew, and cloud ....
Issy is often likened to objects that participate in the water cycle other than the river and sea: dew, cloud, rain, puddles, tears, drops of urine etc. I don't think she is likened to wind, but I stand to be corrected.
What does the event suggest and signify?
On a simple level, the event suggests that even though Nuvoletta might be so sad that she wishes to die, her mother can heal "her heart [that] was brook". Because that is what mothers do. It signifies "death" leading to "rebirth" via Anna.
But, every Anna is/was an Issy. Anna has such desperate feelings, only worse: everyone depends on her for rebirth. What happens when Anna leaves?
I am passing out. O
bitter ending! I’ll slip away before they’re up. They’ll never see.
Nor know. Nor miss me. (627.33)
The tear must become a river.