▁complex ▁models , ▁account ▁for ▁this ▁variations . ▁ ▁Some ▁examples ▁ ▁Depending ▁on ▁the ▁model , ▁some ▁gas ▁properties ▁may ▁be ▁treated ▁as ▁constant ▁with ▁respect ▁to ▁alt itude . ▁ ▁Ocean ▁example ▁If ▁the ▁density ▁of ▁a ▁gas ▁is ▁persistent , ▁then ▁it ▁isn ' t ▁really ▁behav ing ▁like ▁a ▁gas . ▁ ▁Instead ▁it ▁is ▁behav ing ▁like ▁an ▁in com press ible ▁fluid , ▁or ▁liquid , ▁and ▁this ▁situation ▁looks ▁more ▁like ▁an ▁ocean . ▁ ▁Assuming ▁density ▁is ▁constant , ▁then ▁a ▁graph ▁of ▁pressure ▁vs ▁alt itude ▁will ▁have ▁a ▁retained ▁slope , ▁since ▁the ▁weight ▁of ▁the ▁ocean ▁over ▁head ▁is ▁directly ▁proportional ▁to ▁its ▁depth . ▁ ▁Is oth erm al - bar otrop ic ▁approximation ▁and ▁scale ▁height ▁ ▁This ▁atmos pher ic ▁model ▁assumes ▁both ▁mole cular ▁weight ▁and ▁temperature ▁are ▁constant ▁over ▁a ▁wide ▁range ▁of ▁alt itude . ▁Such ▁a ▁model ▁may ▁be ▁called ▁is oth erm al ▁( constant ▁temperature ). ▁In ser ting ▁constant ▁mole cular ▁weight ▁and ▁constant ▁temperature ▁into ▁the ▁equation ▁for ▁the ▁ideal ▁gas ▁law ▁produces ▁the ▁result ▁that ▁density ▁and ▁pressure , ▁the ▁two ▁remaining ▁variables , ▁depend ▁only ▁on ▁each ▁other . ▁ ▁For ▁this ▁reason , ▁this ▁model ▁may ▁also ▁be ▁called ▁bar otrop ic ▁( dens ity ▁depends ▁only ▁on ▁pressure ). ▁ ▁For ▁the ▁is oth erm al - bar otrop ic ▁model , ▁density ▁and ▁pressure ▁turn ▁out ▁to ▁be ▁exponential ▁functions ▁of ▁alt itude . ▁ ▁The