Left: maps of sonic Mach number inferred from optical cirrus map illustrated with a kernel scale of d_k = 10′. The circular kernel is illustrated with the purple disk. Right: mean H I column density fields extracted with the same kernel in the upper panels. Regions with ℳ_s < 0.5 and > 1.5 in the upper panels are indicated by red and cyan contours, respectively, which aggregate at edges of cirrus filaments or patches.