Sunlit raindrops cause a series of light scattering effects. Although they are too large to produce coronas or glories, there is the ascending order of rainbows, starting with the well-known primary and secondary rainbows. Additionally, there is a diffuse concentration of light in the direction of the sun, whose intensity gradually decreases with increasing angular distance.
This forward scattering occurs with light rays that, unlike the rainbows, are not reflected within the drop but are only refracted upon entry and exit. Isaac Newton speculated that this could create a rainbow of "zero order", which was later disproved. For these paths of rays, there is no extreme deflection, meaning there is no sharp concentration of light with color splitting at a specific angle from the sun. However, the term "zero order" entered the literature this way.
To observe the effect, rain must fall in the direction of the sun, without being in the shadow of clouds or self-shadowing, as can happen with very intense showers. When the sun is low, it is geometrically simpler for long paths of light to form through rain shower curtains, intensifying the effect. The scattered light takes on the color of the incoming sunlight, meaning at sunrise or sunset, an intense red "glow" can be observed. Forward scattering also occurs with the sun higher in the sky, and in such cases, the Zero Order Glow appears yellow or white. Strictly speaking, the term "glow" refers only to the red glow when the sun is low. Visually, the Zero Order Glow differs from the afterglow in that the rain itself becomes red, not the clouds. Also, the Zero Order Glow quickly disappears after sunset (similar to the “Red Rainbow”).
Incidentally, it is not mandatory that it rains directly on the observer. Depending on the light and cloud direction, the shower may have already passed, be approaching, or completely miss the observer. Usually, the limited extent of the shower (from the observer's perspective, i.e., the covered solid angle) defines the glow on the sides. Occasionally, almost the entire scattered light disc is visible, or at least the part above the horizon for ground-based observers. From mountains, airplanes, in drone footage, etc., the Zero Order Glow can extend below the horizon.
For hunters of the 3rd and 4th order rainbows, the Zero Order Glow is more of an annoyance. It represents the unavoidable background of scattered light that makes visual observation almost impossible and requires intensive image filtering for photographic evidence.
