( ( During ( the day ) ) ( , ( ( ( ( ( ( the balloons ) ( were ( heated ( by ( the sun ) ) ) ) ) , ) and ) ( ( at night ) ( , ( they ( were ( heated ( by ( ( trapping ( thermal radiation ) ) ( from ( the ( warmer ( , ( lower ( Earth surface ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (PP (IN During) (NP (DT the) (NN day))) (, ,) (S (NP (DT the) (NNS balloons)) (VP (VBD were) (VP (VBN heated) (PP (IN by) (NP (DT the) (NN sun)))))) (, ,) (CC and) (S (PP (IN at) (NP (NN night))) (, ,) (NP (PRP they)) (VP (VBD were) (VP (VBN heated) (PP (IN by) (S (VP (VBG trapping) (NP (JJ thermal) (NN radiation)) (PP (IN from) (NP (DT the) (JJR warmer) (, ,) (JJR lower) (NNP Earth) (NN surface))))))))) (. .)))
During the day, the balloons were heated by the sun, and at night, they were heated by trapping thermal radiation from the warmer, lower Earth surface.
(ROOT (S (NP (NNP Heat)) (VP (VBZ travels) (PP (IN from) (NP (DT the) (NN sun))) (PP (TO to) (NP (NP (DT the) (NN earth) (JJ thermal) (NN radiation)) (PP (IN from) (NP (DT the) (NN sun)))))) (. .)))