.   5 4 6 ) ▁ ▁for ▁simplicity , ▁if ▁it ▁is ▁des irable ▁to ▁avoid ▁computing ▁: ▁ ▁Where : ▁▁ ▁= ▁equivalent ▁temperature ▁▁ ▁= ▁specific ▁gas ▁constant ▁for ▁air ▁( 2 8 7 . 0 4 ▁J /( kg · K )) ▁ ▁Us age ▁▁ ▁This ▁applies ▁on ▁the ▁syn opt ic ▁scale ▁for ▁character isation ▁of ▁air ▁masses . ▁For ▁instance , ▁in ▁a ▁study ▁of ▁the ▁North ▁American ▁Ice ▁Storm ▁of ▁ 1 9 9 8 , ▁profess ors ▁Gy ak um ▁( Mc G ill ▁University , ▁Montreal ) ▁and ▁Ro eb ber ▁( W is consin ▁University , ▁Mil w au kee ) ▁have ▁demonstrated ▁that ▁the ▁air ▁masses ▁involved ▁origin ated ▁from ▁high ▁Ar ctic ▁at ▁an ▁alt itude ▁of ▁ 3 0 0 ▁to ▁ 4 0 0   h Pa ▁the ▁previous ▁week , ▁went ▁down ▁toward ▁the ▁surface ▁as ▁they ▁moved ▁to ▁the ▁Trop ics , ▁then ▁moved ▁back ▁up ▁along ▁the ▁Mississippi ▁Valley ▁toward ▁the ▁St . ▁Lawrence ▁Valley . ▁The ▁back ▁traject ories ▁were ▁evaluated ▁using ▁the ▁constant ▁equivalent ▁potential ▁temper atures . ▁ ▁In ▁the ▁mes osc ale , ▁equivalent ▁potential ▁temperature ▁is ▁also ▁a ▁useful ▁measure ▁of ▁the ▁static ▁stability ▁of ▁the ▁uns atur ated ▁atmosphere . ▁Under ▁normal , ▁st ably ▁strat ified ▁conditions , ▁the ▁potential ▁temperature ▁increases ▁with ▁height , ▁▁▁▁ ▁and ▁vertical ▁mot ions ▁are ▁suppress ed . ▁If ▁the ▁equivalent ▁potential ▁temperature ▁decre ases ▁with ▁height , ▁ ▁the ▁atmosphere ▁is