▁density ▁of ▁the ▁fluid ▁at ▁a ▁fixed ▁reference ▁pressure . ▁ ▁For ▁an ▁ideal ▁gas ▁( see ▁gas ▁laws ), ▁the ▁stability ▁criter ion ▁for ▁an ▁air ▁column ▁is ▁that ▁potential ▁temperature ▁increases ▁monot on ically ▁with ▁height . ▁ ▁To ▁understand ▁this , ▁consider ▁dry ▁con ve ction ▁in ▁the ▁atmosphere , ▁where ▁the ▁vertical ▁variation ▁in ▁pressure ▁is ▁substantial ▁and ▁ad i ab atic ▁temperature ▁change ▁is ▁important : ▁As ▁a ▁par cel ▁of ▁air ▁moves ▁up ward , ▁the ▁ambient ▁pressure ▁dro ps , ▁causing ▁the ▁par cel ▁to ▁expand . ▁ ▁Some ▁of ▁the ▁internal ▁energy ▁of ▁the ▁par cel ▁is ▁used ▁up ▁in ▁doing ▁the ▁work ▁required ▁to ▁expand ▁against ▁the ▁atmos pher ic ▁pressure , ▁so ▁the ▁temperature ▁of ▁the ▁par cel ▁dro ps , ▁even ▁though ▁it ▁has ▁not ▁lost ▁any ▁heat . ▁ ▁Con vers ely , ▁a ▁s inking ▁par cel ▁is ▁com pressed ▁and ▁becomes ▁war mer ▁even ▁though ▁no ▁heat ▁is ▁added . ▁ ▁Air ▁at ▁the ▁top ▁of ▁a ▁mountain ▁is ▁usually ▁col der ▁than ▁the ▁air ▁in ▁the ▁valley ▁below , ▁but ▁the ▁arrangement ▁is ▁not ▁un stable : ▁if ▁a ▁par cel ▁of ▁air ▁from ▁the ▁valley ▁were ▁somehow ▁lifted ▁up ▁to ▁the ▁top ▁of ▁the ▁mountain , ▁when ▁it ▁arrived ▁it ▁would ▁be ▁even ▁col der ▁than ▁the ▁air ▁already ▁there , ▁due ▁to ▁ad i ab atic ▁cool ing ; ▁it ▁would ▁be ▁heav ier ▁than ▁the ▁ambient ▁air , ▁and ▁would ▁sink ▁back ▁toward ▁its ▁original