Type III Billows

(Section 2.6.3.3.5)

These are arrangements of closely spaced, roughly parallel short streaks. The distance separating adjacent billows ranges from about 1 km to 10 km. Billows sometimes lie across the long bands, giving the appearance of a comb or feather. At other times, they appear alone against the veil background. The billows may change their form and arrangement, or appear and disappear within several minutes, much more rapidly and frequently than the long bands. This noctilucent cloud type may also be divided into two groups:

IIIa: Comprised of short, straight and narrow streaks

IIIb: Exhibit a wave-like structure with undulations