This is the 7256th most frequent Russian word.
воздушно
Airily.
"Воздушно" in this case characterizes the sky's appearance, suggesting it is light and delicate, like air.
Небо было настолько воздушно, что казалось не реальным.
The sky was so airy that it seemed unreal.
Here, "воздушно" describes the nature of the dancing, implying lightness and grace.
Девушка танцевала так воздушно, будто парила над землёй.
The girl danced so airily as if floating above the ground.
In this usage, "воздушно" is part of a compound term describing the texture of the cake, meaning light and fluffy.