▁inputs ▁besides ▁alt itude . ▁ ▁Basic ▁assumptions ▁The ▁gas ▁which ▁compr ises ▁an ▁atmosphere ▁is ▁usually ▁assumed ▁to ▁be ▁an ▁ideal ▁gas , ▁which ▁is ▁to ▁say : ▁ ▁Where ▁ ρ ▁is ▁mass ▁density , ▁M ▁is ▁average ▁mole cular ▁weight , ▁P ▁is ▁pressure , ▁T ▁is ▁temperature , ▁and ▁R ▁is ▁the ▁ideal ▁gas ▁constant . ▁ ▁The ▁gas ▁is ▁held ▁in ▁place ▁by ▁so - called ▁" h ydro static " ▁forces . ▁▁ ▁That ▁is ▁to ▁say , ▁for ▁a ▁particular ▁layer ▁of ▁gas ▁at ▁some ▁alt itude : ▁the ▁down ward ▁( t ow ards ▁the ▁planet ) ▁force ▁of ▁its ▁weight , ▁the ▁down ward ▁force ▁ex ert ed ▁by ▁pressure ▁in ▁the ▁layer ▁above ▁it , ▁and ▁the ▁up ward ▁force ▁ex ert ed ▁by ▁pressure ▁in ▁ ▁the ▁layer ▁below , ▁all ▁sum ▁to ▁zero . ▁ ▁Mathemat ically ▁this ▁is : ▁ ▁Finally , ▁these ▁variables ▁describing ▁the ▁system ▁do ▁not ▁change ▁with ▁time ; ▁i . e . ▁it ▁is ▁a ▁static ▁system . ▁ ▁g _ 0 , ▁gravit ational ▁acceleration ▁is ▁used ▁here ▁as ▁a ▁constant , ▁with ▁same ▁value ▁as ▁standard ▁gravity ▁( aver age ▁acceleration ▁due ▁to ▁gravity ▁on ▁the ▁surface ▁of ▁the ▁Earth ▁or ▁other ▁big ▁body ). ▁For ▁the ▁basis ▁of ▁simplicity ▁it ▁doesn ' t ▁vary ▁with ▁latitude , ▁alt itude ▁or ▁location . ▁The ▁variation ▁due ▁to ▁all ▁these ▁factors ▁is ▁about ▁ 1 % ▁up ▁to ▁ 5 0 km . ▁More