air hoar; soft rime; air frost
Meteorological term for a feathery ice deposit formed on trees and other objects by direct sublimation of water vapor, typically in cold, calm conditions.
Soft rime often forms on tree branches on cold, calm mornings.
Compound of 樹 (tree) and 霜 (frost), literally 'tree frost'. The reading じゅそう is the on'yomi for both characters.