As the photographs show, afterglows vary markedly in appearance depending upon the depth and height of the stratospheric clouds in the observer's vicinity. Color and intensity is also affected by the amount of haze and tropospheric cloudiness along the path of light reaching the stratosphere. The cirrus streaks in the foreground have long since become shaded, but in the center of the view, a distant tropospheric cloud tower below the horizon is casting a dark shadow across the afterglow. Blue light scattered downward through the thin cloud producing the afterglow, mixed with the red light which illuminates it, is responsible for the lilac hue. The nearer parts are being illuminated by light which has passed through the troposphere and is therefore strongly reddened.
More direct sunlight illuminates the brighter region close to the horizon.
The preceding pages are only an brief introduction to the physics and meteorology of the twilight sky. Increased understanding of the nature sunrise and sunset colors can only increase our appreciation of them.