Heat capacity ratio of air
| Value | 1.4 — |
| Status | Conventional / typical value |
| Source | NIST Chemistry WebBook; ASHRAE Handbook-Fundamentals |
| Categories | ThermodynamicEngineering & Trade |
| part per trillion | 1,400,000,000,000 ppt |
| part per billion | 1,400,000,000 ppb |
| part per million | 1,400,000 ppm |
| per cent mille | 140,000 pcm |
| basis point | 14,000 bp |
| per mille | 1,400 per mille |
| percent | 140 % |
| ratio | 1.4 — |
Learning zone
The ratio of specific heats governs what happens when a gas is compressed too fast to exchange heat: pV^γ stays constant, and the temperature rises. It is why a bicycle pump gets hot, why diesel engines ignite without a spark, and why the discharge of an air compressor needs an aftercooler. It also sets the speed of sound, a = √(γRT), giving 343 m/s in air at 20 °C.
The value 1.400 is not arbitrary. Nitrogen and oxygen are diatomic, with three translational and two rotational degrees of freedom active at room temperature, so equipartition predicts γ = (5+2)/5 = 1.4 exactly — one of the cleanest confirmations of kinetic theory. It falls slowly as temperature rises and vibrational modes wake up: about 1.40 at 20 °C, 1.35 at 500 °C, 1.30 at 1000 °C.