The equilibrium air module uses the curve fits of Tannehill, et al. to calculate the thermodynamic and molecular properties of air when intermolecular forces become important. As temperature becomes large, dissociation and eventually ionization occurs. The thermodynamic properties of air become dependent on two variables (T = T(P,e)). For hypersonic flows, this has the effect of weakening the strength of a shock wave. The table below shows the temperatures at which dissociation and ionization become important. Molecular vibrations start occurring around T=600 K which is the temperature value where starts γ changing.
| Temperature Range | Chemical Composition |
| T < 2500 K | 78% N2, 21% O2 |
| 2500 K < T < 4000 K | Oxygen dissociation regime. Some NO is formed |
| 4000 K < T < 8000 K | Nitrogen dissociation regime. Oxygen is fully dissociated. |
| 8000 K < T | Ionization of atomic constituents occurs |
