( ( During ( a ( black frost ) ) ) ( , ( ( ( ( few or ) no ) ( ice crystals ) ) ( ( form ( because ( ( ( the air ) ( in ( the ( lower atmosphere ) ) ) ) ( is ( too dry ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN During) (NP (DT a) (JJ black) (NN frost))) (, ,) (NP (ADJP (JJ few) (CC or) (DT no)) (NN ice) (NNS crystals)) (VP (VBP form) (SBAR (IN because) (S (NP (NP (DT the) (NN air)) (PP (IN in) (NP (DT the) (JJR lower) (NN atmosphere)))) (VP (VBZ is) (ADJP (RB too) (JJ dry)))))) (. .)))
During a black frost , few or no ice crystals form because the air in the lower atmosphere is too dry.
(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))))) (. .)))