( It ( also ( ( ( ( ( happens ( during ( ( the ( natural process ) ) ( of convection ) ) ) ) ( as ( ( warm air ) ( ( ( rises and ) escapes ) ( through cracks ) ) ) ) ) , ) ( ( causing ( cold air ) ) ( to ( be ( pulled ( into ( ( the ( lower portion ) ) ( of ( the house ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (PRP It)) (ADVP (RB also)) (VP (VBZ happens) (PP (IN during) (NP (NP (DT the) (JJ natural) (NN process)) (PP (IN of) (NP (NN convection))))) (SBAR (IN as) (S (NP (JJ warm) (NN air)) (VP (VBZ rises) (CC and) (VBZ escapes) (PP (IN through) (NP (NNS cracks)))))) (, ,) (S (VP (VBG causing) (NP (JJ cold) (NN air)) (S (VP (TO to) (VP (VB be) (VP (VBN pulled) (PP (IN into) (NP (NP (DT the) (JJR lower) (NN portion)) (PP (IN of) (NP (DT the) (NN house)))))))))))) (. .)))
It also happens during the natural process of convection as warm air rises and escapes through cracks, causing cold air to be pulled into the lower portion of the house.
(ROOT (S (NP (NP (DT The) (JJR lower) (NN density)) (PP (IN of) (NP (JJ warm) (NN air)))) (VP (VBZ causes) (S (NP (PRP it)) (VP (TO to) (VP (VB rise) (PP (IN above) (NP (JJ cold) (NN air))))))) (. .)))