In addition, we switch to the Celsius temperature θ = T − 273.15 K, which is useful to calculate air speed in the region near 0 °C (273 K). Then, for dry air, c a i r = γ γ ⋅ ⋅ R ∗ ∗ ⋅ ⋅ T = γ γ ⋅ ⋅ R ∗ ∗ ⋅ ⋅ ( θ θ + 273.15 K ) , c a i r = γ γ ⋅ ⋅ R ∗ ∗ ⋅ ⋅ 273.15 K ⋅ ⋅ 1 + θ θ 273.15 K .