Zero Order Glow
When rain is falling with the setting sun behind it, there sometimes appears a golden or deep red glow in shape of a semicircle around the sun. Literature calls this glow “Zero Order Glow”. The name means that this glow is a zero order rainbow. This is because the light is not reflected within the raindrops once or twice, as it is the case in primary and secondary rainbows. In the case of a zero order glow, the light passes through the raindrops without being reflected, leaving them only a bit deviated. So there is no bow-shaped concentration of light, and the zero order glow appears in form of a diffuse shining area around the sun.
In normal cases, the phenomenon is visible only above the horizon. But when this photographs (2 –3) was taken, a fine drizzle fell into the valley from a very low layer of clouds, causing also a glow beneath the horizon. Due to the low sun elevation and the long way the sunlight had to travel through the atmosphere, together with the additional light diffraction on the small drizzle particles, there is only the red light visible.
Author: Claudia Hinz, Wendelstein (1835m), Germany