( ( If ( ( the site ) ( is ( located ( on ( a mountain ) ) ) ) ) ) ( ( range ( ( oriented perpendicularly ) ( to ( ( the ( prevailing wind ) ) ( ( the air ) ( is ( forced ( to ( flow ( over ( the ( mountain barrier ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (SBAR (IN If) (S (NP (DT the) (NN site)) (VP (VBZ is) (VP (VBN located) (PP (IN on) (NP (DT a) (NN mountain))))))) (VP (VBP range) (ADJP (VBN oriented) (RB perpendicularly) (PP (TO to) (NP (NP (DT the) (VBG prevailing) (NN wind)) (SBAR (S (NP (DT the) (NN air)) (VP (VBZ is) (VP (VBN forced) (S (VP (TO to) (VP (VB flow) (PP (IN over) (NP (DT the) (NN mountain) (NN barrier)))))))))))))) (. .)))
If the site is located on a mountain range oriented perpendicularly to the prevailing wind the air is forced to flow over the mountain barrier.
(ROOT (S (NP (NN Chinook) (NNS winds)) (VP (VBP occur) (SBAR (WHADVP (WRB when)) (S (NP (NN air)) (VP (VBZ is) (VP (VBN forced) (PP (IN over) (NP (DT a) (NN mountain) (NN range)))))))) (. .)))