( ( ( Warm summers ) ( in ( the ( northern hemisphere ) ) ) ) ( ( occur ( when ( ( ( ( that hemisphere ) ( is ( tilted ( toward ( the sun ) ) ) ) ) and ) ( ( the Earth ) ( is ( nearest ( ( the sun ) ( in ( its ( elliptical orbit ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (JJ Warm) (NNS summers)) (PP (IN in) (NP (DT the) (JJ northern) (NN hemisphere)))) (VP (VBP occur) (SBAR (WHADVP (WRB when)) (S (S (NP (DT that) (NN hemisphere)) (VP (VBZ is) (VP (VBN tilted) (PP (IN toward) (NP (DT the) (NN sun)))))) (CC and) (S (NP (DT the) (NN Earth)) (VP (VBZ is) (VP (VBG nearest) (NP (NP (DT the) (NN sun)) (PP (IN in) (NP (PRP$ its) (JJ elliptical) (NN orbit)))))))))) (. .)))
Warm summers in the northern hemisphere occur when that hemisphere is tilted toward the sun and the Earth is nearest the sun in its elliptical orbit.
(ROOT (S (NP (NP (DT The) (NN tilt)) (PP (IN of) (NP (NP (NN earth)) (PP (IN on) (NP (PRP$ its) (NNS axis)))))) (VP (VBZ causes) (NP (NN summer)) (S (VP (TO to) (VP (VB be) (VP (VBN warmer) (PP (IN than) (NP (NP (NN winter)) (PP (IN in) (NP (DT the) (JJ northern) (NN hemisphere))))) (SBAR (IN because) (S (NP (NP (DT the) (NNS rays)) (PP (IN of) (NP (DT the) (NN sun)))) (VP (VBP strike) (NP (DT the) (JJ northern) (NN hemisphere)) (ADVP (ADVP (RBR more) (RB directly)) (PP (IN in) (NP (DT the) (NN summer)))))))))))) (. .)))