( ( FROST ( ( ( The covering ) ( of ( ice crystals ) ) ) ( that ( forms ( by ( ( direct sublimation ) ( on ( ( exposed surfaces ) ( ( whose temperature ) ( is ( below freezing ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB FROST) (NP (NP (DT The) (NN covering)) (PP (IN of) (NP (NN ice) (NNS crystals))) (SBAR (WHNP (WDT that)) (S (VP (VBZ forms) (PP (IN by) (NP (NP (JJ direct) (NN sublimation)) (PP (IN on) (NP (NP (VBN exposed) (NNS surfaces)) (SBAR (WHNP (WP$ whose) (NNS temperature)) (S (VP (VBZ is) (PP (IN below) (NP (NN freezing))))))))))))))) (. .)))
FROST The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.
(ROOT (S (NP (NP (DT The) (NN ice) (NNS crystals)) (SBAR (WHNP (WDT that)) (S (VP (VBP form) (PP (IN on) (NP (DT the) (NN ground))))))) (VP (VBP are) (VP (VBN called) (S (ADJP (JJ frost))))) (. .)))