▁less ▁dense ▁until ▁they ' re ▁so ▁thin ▁that ▁they ' re ▁space . ▁ ▁The ▁U . S . ▁Standard ▁At mos phere ▁ ▁The ▁U . S . ▁Standard ▁At mos phere ▁model ▁starts ▁with ▁many ▁of ▁the ▁same ▁assumptions ▁as ▁the ▁is oth erm al - bar otrop ic ▁model , ▁including ▁ideal ▁gas ▁behavior , ▁and ▁constant ▁mole cular ▁weight , ▁but ▁it ▁diff ers ▁by ▁defining ▁a ▁more ▁real istic ▁temperature ▁function , ▁consisting ▁of ▁eight ▁data ▁points ▁connected ▁by ▁straight ▁lines ; ▁i . e . ▁regions ▁of ▁constant ▁temperature ▁gradient . ▁( See ▁graph .) ▁▁ ▁Of ▁course ▁the ▁real ▁atmosphere ▁does ▁not ▁have ▁a ▁temperature ▁distribution ▁with ▁this ▁exact ▁shape . ▁ ▁The ▁temperature ▁function ▁is ▁an ▁approximation . ▁ ▁Val ues ▁for ▁pressure ▁and ▁density ▁are ▁then ▁calculated ▁based ▁on ▁this ▁temperature ▁function , ▁and ▁the ▁constant ▁temperature ▁grad ients ▁help ▁to ▁make ▁some ▁of ▁the ▁math s ▁easier . ▁ ▁NASA ▁Global ▁Reference ▁At mos pher ic ▁Model ▁ ▁The ▁NASA ▁Earth ▁Global ▁Reference ▁At mos pher ic ▁Model ▁( E arth - G RAM ) ▁was ▁developed ▁by ▁the ▁Marshall ▁Space ▁Fl ight ▁Center ▁to ▁provide ▁a ▁design ▁reference ▁atmosphere ▁that , ▁unlike ▁the ▁standard ▁atmos pher es , ▁allows ▁for ▁ge ographical ▁vari ability , ▁a ▁wide ▁range ▁of ▁alt itudes ▁( sur face ▁to ▁orb ital ▁alt itudes ), ▁and ▁different ▁months ▁and ▁times ▁of ▁day . ▁ ▁It ▁can ▁also ▁simulate ▁spatial ▁and ▁temporal ▁perturb ations ▁in ▁atmos pher