Effective scale height of the disk, relative to the local background value H₀(r). a): case of the giant vortex of Fig. 8. The scale height is 31% larger than H₀(r) in the vortex centre but can reach 50% in the outer part of the spiral arm because of the shock wave. b): case of the large incompressible vortex of Fig. 11 (top). In the centre of the vortex the scale height is 14% larger than the background value. c): case of the compressible vortex of Fig. 11 (bottom). Even with a smaller density maximum, the scale height is about 10% larger than the background, owing to a warmer temperature in this vortex; there is also a slight increase in the spiral waves.