Type I Veils

(Section 2.6.3.3.3)

These are very tenuous, lack well-defined structure and are often present as a background to other forms. They resemble Cirrus clouds, occasionally contain faintly fibrous structure, and often exhibit a flickering luminosity. Veils are the simplest form of noctilucent clouds and often precede (by about half an hour) the appearance of noctilucent clouds with well-defined structure.