Purple light

(Section 3.2.3.11.1)

In the direction of the Sun, a glow called purple light is sometimes seen. This appears as a segment of a large luminous disc and extends upwards from the horizon. The purple light rises gradually, reaching a maximum both in size and luminance when the Sun is 3° or 4° below the horizon. It then descends and disappears when the Sun is about 6° below the horizon (at the end of civil twilight). Occasionally, when this first purple light has disappeared, the phenomenon repeats itself with less intensity. The second patch of purple light appears at a slightly lower elevation than the first, but otherwise follows the same sequence.