Sentence:  He looked at the sky and saw the white cumulus built like friendly piles of ice cream and high above where the thin feathers of the <WSD>cirrus</WSD> against the high September sky. 
Meaning of the word marked with <WSD> tags:  A principal high-level cloud type characterised by white, delicate filaments or wisps, of white (or mostly white) patches, or of narrow bands, found at an altitude of above 7000 metres.