1 A form of aurora, consisting of a very tenuous glow, lacking any structure, which may appear over a wide area of sky and as a background to other forms.
2 The simplest type of noctilucent cloud (Type I), consisting of thin, tenuous cloud, sometimes with faint fibrous structure. This form often precedes the main display, which usually exhibits more definite types, such as bands (2), billows (2), and whirls (2).
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