These molecules made their last collision at a distance l cos ⁡ ⁡ θ θ above and below the gas layer, and each will contribute a molecular kinetic energy of ε ε ± ± = ( ε ε 0 ± ± m c v l cos ⁡ ⁡ θ θ d T d y ) , where c v is the specific heat capacity. Again, plus sign applies to molecules from above, and minus sign below. Note that the temperature gradient d T / d y can be considered to be constant over a distance of mean free path.