( ( ( A ( total amount ) ) ( of ( ( thermal energy ) ( ( received ( per ( unit area ) ) ) ( at ( ( a ( specified distance ) ) ( from ( a ( nuclear explosion ) ) ) ) ) ) ) ) ) ( ( ( is generally ) ( expressed ( in ( terms ( of ( calories ( per ( square centimeter ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT A) (JJ total) (NN amount)) (PP (IN of) (NP (NP (JJ thermal) (NN energy)) (VP (VBN received) (PP (IN per) (NP (NN unit) (NN area))) (PP (IN at) (NP (NP (DT a) (VBN specified) (NN distance)) (PP (IN from) (NP (DT a) (JJ nuclear) (NN explosion))))))))) (VP (VBZ is) (ADVP (RB generally)) (VP (VBN expressed) (PP (IN in) (NP (NP (NNS terms)) (PP (IN of) (NP (NP (NNS calories)) (PP (IN per) (NP (JJ square) (NN centimeter))))))))) (. .)))
A total amount of thermal energy received per unit area at a specified distance from a nuclear explosion is generally expressed in terms of calories per square centimeter.
(ROOT (S (NP (DT A) (NN calorie)) (VP (VBZ is) (VP (VBN used) (S (VP (TO to) (VP (VB express) (NP (NP (DT the) (NN amount)) (PP (IN of) (NP (NN energy)))) (PP (IN in) (NP (NN food)))))))) (. .)))
A calorie is used to express the amount of energy in food.