Since the air is dominated by diatomic gases (with nitrogen and oxygen contributing about 99%), its molar internal energy is close to c v T = (5/2) R T, determined by the 5 degrees of freedom exhibited by diatomic gases. See the graph at right. For 140 K < T < 380 K, c v differs from (5/2) R d by less than 1%. Only at temperatures well above temperatures in the troposphere and stratosphere do some molecules have enough energy to activate the vibrational modes of N 2 and O 2. The specific heat at constant volume, c v, increases slowly toward (7/2) R as temperature increases above T = 400 K, where c v is 1.3% above (5/2) R d = 717.5 J/(K kg).