Rainbows - Zero Order Glow

Last updated on September 16, 2023

Rainbows - Zero Order Glow: A Mystical Phenomenon in the Sky

Rainbows have always captivated our imagination with their vibrant colors and ethereal beauty. But there is more to the atmospheric optics of rainbows than meets the eye. In addition to the primary and secondary rainbows, there is another phenomenon known as the "zero order glow" that adds an intense glow to the sky opposite the sun. This mystical occurrence occurs when sunlight passes through raindrops without any internal reflection, creating a diffuse light that lacks the sharp intensity concentration of a rainbow's rim.

When you next encounter a rainbow, take a moment to look towards the sun. If rain is falling between you and the sun, you may witness this extraordinary glow. The best time to observe it is when the sun is low in the sky, already displaying hues of orange or red. The zero order glow is particularly pronounced during these conditions.

The zero order glow is formed as light passes through raindrops and emerges on the other side without undergoing internal reflection. Unlike rainbows, which result from multiple reflections within raindrops, this phenomenon involves no reflections at all. Hence, its name "zero order glow." The light that emerges from the raindrops creates a diffuse illumination that bathes the sky in an intense orange hue.

To fully appreciate the zero order glow, it is essential to understand its unique characteristics. Here are some key points about this intriguing atmospheric phenomenon:

1. Absence of Internal Reflection: Unlike rainbows, where light undergoes multiple internal reflections within raindrops, the zero order glow occurs due to the absence of internal reflection. This absence leads to a diffuse light that lacks the concentrated intensity found in rainbows.

2. Sunward Direction: The zero order glow is most prominent in the direction of the sun. As sunlight passes through raindrops and emerges on the other side, it creates an intense glow in the sky opposite the sun.

3. Intense Orange Color: The zero order glow manifests as a vibrant orange hue that suffuses the sky. This color is a result of the interaction between sunlight and the raindrops.

4. Low Sun Angle: The zero order glow is best observed when the sun is low in the sky. During these times, the sunlight has to traverse a longer path through the atmosphere, enhancing the chances of encountering raindrops and creating the glow.

5. Enhancement by Fine Rain: Fine rain droplets play a crucial role in intensifying the zero order glow. When fine rain is falling between the observer and the sun, it further enhances the glow's brilliance.

6. Unique Optical Phenomenon: The zero order glow is distinct from rainbows and other atmospheric optical phenomena. Its diffuse and ethereal nature adds an enchanting element to the sky's appearance during rain showers.

The zero order glow is a reminder of the countless wonders that exist within our natural world. Its presence during rain showers adds an extra layer of magic to an already mesmerizing display of colors. So, next time you witness a rainbow, don't forget to turn your gaze towards the sun and immerse yourself in the mystical beauty of the zero order glow.

Early evening rain shower.There were bright primary and secondary rainbows opposite the sun.Sunward, where fine rain was also falling, the sky was suffused by an intense orange. The sun was the same deep colour and had hazy blurred edges, the small inset image does not do it justice.The unusual glow was caused by sunlight passing through raindrops without any internal reflection. Image©2003 Les Cowley.

Next time there is a rainbow look at the sky towards the sun. If rain is falling between you and the sun there will likely be an intense glow. It is best seen when the sun is low and already orange or red.This extra glow, over and above the ordinary sky brightness in the sun's direction, is formed by light passing through the raindrops and emerging the other side without having been internally reflected. I call this the 'zero order glow' zero because the number of reflections involved is zero, glow because it is a diffuse light without the sharp intensity concentration of a rainbow's rim.

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Reference Atmospheric Optics

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