These are long streaks, often occurring in groups arranged roughly parallel to each other or interwoven at small angles, but occasionally an isolated band is observed. Two groups of this type occur:
IIa: Comprised of streaks with diffuse, blurred edges
IIb: Have sharply defined edges
Bands show little change in location at low elevation angles, but changes in brightness can occur in periods of 20−60 minutes within the general structure. Blurred bands with little mobility are often the predominant structure in the noctilucent cloud field, particularly when the brightness is low. Smaller streaks with twists or bends may lie across the bands or branch out from them. Distances of 10−60 km separating successive bands are common and values of over 100 km have been reported.