Frost-fall builds featherlike ice formations only in rare atmosphere and intense cold

Evidence

That curious phenomena called frost-fall was occurring, in which, whatever moisture may exist in the air, somehow aggregates into feathers and fern leaves, the loveliest of creations, only seen in rarefied air and intense cold. One breath and they vanish. The air was filled with diamond sparks quite intangible. — Isabella L. Bird, A Lady’s Life in the Rocky Mountains, 565

Categorization

• DEFINE: Explains frost-fall, a rare winter phenomenon of air moisture aggregating into fern-like ice shapes.
• CANDY: Frost-fall — ice feathers that vanish at one breath, seen only in rarefied air and extreme cold — is a perfect dinner-party nature fact.