"There is not a cloud in the sky."
Translation:Il n'y a pas un nuage dans le ciel.
Both the French and the English expressions are logically poor here, because "not one cloud" could mean either zero or more than one cloud (unless one adds "even" as Sitesurf has). I guess the fact that dapertra's translation isn't the only accepted one is disappointing to those of us who were expecting French to carry extra precision.
Nope, sorry. "au ciel" would only be used to mark the idea of a destination ("je monte au ciel" = "I am going to heaven", "Lever les yeux au ciel" = "Look towards the sky"[that is, as a literal translation; the actual meaning is more akin to "rolling your eyes in exasperation]).
When you want to point out something in the sky, "dans" is your preposition.