The gas layer of our Galaxy is known to be warped in a rather asymmetric way. Outside the solar circle, the HI disk reaches ≈ 3 kpc above the plane on one side (in the North), whereas it extends down to only 1 kpc below the plane on the other side (in the South); in the outermost regions, at a galactocentric radius of ≈ 30 kpc, the warp is found to reach the height of 6 kpc. Many galaxies are affected by similar global distortions of the disk (Fig. 19.1; see also Fig. 3.4 in Part I). In a survey of twenty-six edge-on galaxies, twenty were found to be warped in HI, often in an asymmetric way, with the warp usually starting at the edge of the optical disk. Warps also have been noted (although often only as marginal evidence) for the stellar disk. The fact that stars and gas both participate in the warp is an indication that the phenomenon is probably dominated by gravitational forces. The presence of significant warps only in the outer regions, where the stellar disk ends, may simply be due to the small inertia of the disk in the outer parts compared with the heavier inner regions.

Besides the frequent integral-sign appearance (i.e., dominance of an m = 1 relatively open distortion of the disk), the morphology of warps is not as well documented.