▁increase ▁in ▁ ▁alt itude ▁necessary ▁for ▁P ▁or ▁ ρ ▁to ▁drop ▁to ▁ 1 / e ▁of ▁its ▁initial ▁value ▁is ▁called ▁the ▁scale ▁height : ▁ ▁where ▁R ▁is ▁the ▁ideal ▁gas ▁constant , ▁T ▁is ▁temperature , ▁M ▁is ▁average ▁mole cular ▁weight , ▁and ▁g 0 ▁is ▁the ▁gravit ational ▁acceleration ▁at ▁the ▁planet ' s ▁surface . ▁Using ▁the ▁values ▁T = 2 7 3 ▁K ▁and ▁M = 2 9 ▁g / m ol ▁as ▁characteristic ▁of ▁the ▁Earth ' s ▁atmosphere , ▁H ▁= ▁R T / M g ▁= ▁( 8 . 3 1 5 * 2 7 3 )/ ( 2 9 * 9 . 8 ) ▁= ▁ 7 . 9 9 , ▁or ▁about ▁ 8   km , ▁which ▁coinc ident ally ▁is ▁approximate ▁height ▁of ▁M t . ▁Ever est . ▁ ▁For ▁an ▁is oth erm al ▁atmosphere , ▁▁ ▁or ▁about ▁ 6 3 % ▁of ▁the ▁total ▁mass ▁of ▁the ▁atmosphere ▁exists ▁between ▁the ▁planet ' s ▁surface ▁and ▁one ▁scale ▁height . ▁ ▁( The ▁total ▁air ▁mass ▁below ▁a ▁certain ▁alt itude ▁is ▁calculated ▁by ▁integr ating ▁over ▁the ▁density ▁function .) ▁ ▁For ▁the ▁ocean ▁example ▁there ▁was ▁a ▁sharp ▁transition ▁in ▁density ▁at ▁the ▁top ▁or ▁" sur face " ▁of ▁the ▁ocean . ▁ ▁However , ▁for ▁atmos pher es ▁made ▁of ▁gas ▁there ▁is ▁no ▁equivalent ▁sharp ▁transition ▁or ▁edge . ▁ ▁Gas ▁atmos pher es ▁simply ▁get ▁less ▁and