▁during ▁that ▁pressure ▁change . ▁It ▁is ▁therefore ▁more ▁conser ved ▁than ▁the ▁ordinary ▁potential ▁temperature , ▁which ▁remains ▁constant ▁only ▁for ▁uns atur ated ▁vertical ▁mot ions ▁( press ure ▁changes ). ▁▁ ▁is ▁the ▁temperature ▁a ▁par cel ▁of ▁air ▁would ▁reach ▁if ▁all ▁the ▁water ▁v apor ▁in ▁the ▁par cel ▁were ▁to ▁con den se , ▁re le asing ▁its ▁lat ent ▁heat , ▁and ▁the ▁par cel ▁was ▁brought ▁ad i ab atically ▁to ▁a ▁standard ▁reference ▁pressure , ▁usually ▁ 1 0 0 0 ▁h Pa ▁( 1 0 0 0 ▁m bar ) ▁which ▁is ▁roughly ▁equal ▁to ▁atmos pher ic ▁pressure ▁at ▁sea ▁level . ▁ ▁Its ▁use ▁in ▁estim ating ▁atmos pher ic ▁stability ▁ ▁St ability ▁of ▁in com press ible ▁fluid ▁Like ▁a ▁ball ▁bal anced ▁on ▁top ▁of ▁a ▁hill , ▁dens er ▁fluid ▁lying ▁above ▁less ▁dense ▁fluid ▁would ▁be ▁dynamically ▁un stable : ▁over turn ing ▁mot ions ▁( con ve ction ) ▁can ▁lower ▁the ▁center ▁of ▁gravity , ▁and ▁thus ▁will ▁occur ▁sp ont ane ously , ▁rapidly ▁producing ▁a ▁stable ▁strat ification ▁which ▁is ▁thus ▁the ▁observed ▁condition ▁almost ▁all ▁the ▁time . ▁The ▁condition ▁for ▁stability ▁of ▁an ▁in com press ible ▁fluid ▁is ▁that ▁density ▁decre ases ▁monot on ically ▁with ▁height . ▁ ▁St ability ▁of ▁compress ible ▁air : ▁potential ▁temperature ▁If ▁a ▁fluid ▁is ▁compress ible ▁like ▁air , ▁the ▁criter ion ▁for ▁dynamic ▁stability ▁instead ▁involves ▁potential ▁density , ▁the