The author could have used "autumn sky" instead of "autumn air", but the choice is for literary reasons, the "air" in "autumn air" echoing the "air" in the balloons. Similarly, the organizers could have called it The Balloon Festival instead of The Balloon Fiesta, but "fiesta" is a livelier word than than "festival," which is in keeping the the whimsical nature of balloons themselves.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.