Results of the discrete FFT of the HI velocity dispersion map of M 33 (100 pc resolution, in the CNM case). The amplitudes (upper panel) and phases (middle panel) of the Fourier modes are shown as black open circles (k = 0), green open circles (k = 1), filled circles (k = 2), blue upward filled triangles (k = 3), and red open diamonds (k = 4). The reference ϕ = 0 is chosen along the semi-major axis of the NE approaching half of M 33. Bottom panel: σ₂/σ_k, using the same colors and symbols as above for the k = 1, 3, 4 modes. The range of ratios is chosen up to 10 for clarity, but the total range is 78 for σ₂/σ₁ and σ₂/σ₄, and 12 for σ₂/σ₃.