( ( ( ( Precipitation , ) ( which ( is ( associated ( with ( rising air ) ) ) ) ) ) , ) ( ( ( ( will therefore ) not ) ( occur ( unless ( ( a ( low-pressure center ) ) ( passes ( through ( ( ( the region ) , ) ( ( a ( rare phenomenon ) ) ( in ( the summer ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NNP Precipitation)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ is) (VP (VBN associated) (PP (IN with) (NP (VBG rising) (NN air))))))) (, ,)) (VP (MD will) (ADVP (RB therefore)) (RB not) (VP (VB occur) (SBAR (IN unless) (S (NP (DT a) (JJ low-pressure) (NN center)) (VP (VBZ passes) (PP (IN through) (NP (NP (DT the) (NN region)) (, ,) (NP (NP (DT a) (JJ rare) (NN phenomenon)) (PP (IN in) (NP (DT the) (NN summer))))))))))) (. .)))
Precipitation, which is associated with rising air, will therefore not occur unless a low-pressure center passes through the region, a rare phenomenon in the summer.
(ROOT (NP (NP (NP (VBG Rising)) (CC and) (NP (VBG sinking) (NN air) (NNS influences))) (VP (VBN precipitation) (PP (IN in) (NP (DT a) (NN region)))) (. .)))