Santa Fe Supernumerary Rainbow
A brilliant rainbow envelops purple and green supernumerary bows accented with anti-crepuscular rays over the Sangre de Cristo Mountains north of Santa Fe, New Mexico.
Rainbows, Supernumerary Bows and Anticrepuscular Rays, Oh My!
The dark thunderstorm covering the skies above the Sangre de Cristo mountains did not show much promise for rainbows. So I was pleasantly surprised later when I saw a vivid section of a rainbow shining brightly against the stormy indigo sky.
This little section of rainbow packed in some interesting optical effects!Read More
Spokes of dark and light rays crossed the rainbow and converged towards the distant horizon. How could they converge to a point on the eastern horizon, opposite the setting sun in the west? These rays are known as “anti-crepuscular” rays are an optical illusion as they appear to converge on the horizon.
Inside the bright primary rainbow were alternating arcs of pastel violets and greens that faded towards the center of the rainbow. These “supernumerary bows” are caused by the wave nature of light.
The contrast between the anticrepuscular rays and the dark storm clouds was what really inspired me to create this photograph. The bright rainbow initially caught my eye. The anticrepuscular rays add novelty and mystery to the picture.
The Story Behind the Photograph
On this stormy afternoon, I was not expecting to see any rainbows., The sky was dark and dense with clouds. I really wanted it to rain as Northern New Mexico was still in a drought. Any rain or snow was welcome!
When storms develop over the mountains by home, I usually venture out with camera and tripod to see what I can find. Each time is different. Mornings and evenings are the best. I almost stayed home this afternoon since the entire sky was covered in clouds yet lacking drama.Read More
Ironically, it is days like these when I am often rewarded by taking the initiative to “just do it” and see what nature provides.
Exploring the piñon and juniper forest that quickly rises up to higher elevation ponderosa, and eventually aspen and spruce forests of the Sangre de Cristo, I kept a close eye on the dark cloud layer. I was looking for something to accent the uniformly cloudy sky. Perhaps some lightning or cloud shadows would appear, or this would be one those days where no photos are taken. Luckily, Northern New Mexico skies are rarely dull and gray.
Over the Sangre de Cristo, I saw part of a rainbow begin to form. There were two fragments of the rainbow arc, at the base of where a full rainbow would appear. The small rainbows were changing quickly; getting more intense as I hurriedly set up my tripod.
Spokes of dark and bright sunbeams converged towards the eastern horizon. Seeing these through the camera lens excited me and validated that this afternoon’s photography adventure was worthwhile after all. The light inside the rainbow was golden and lit up the landscape. In a few short minutes, the rainbow disappeared.
Catching these moments of stimulating light in nature is an adventure. I have to move quickly before they vanish. Ideally, my planning works out so that a picturesque foreground can be incorporated into the composition. Too many times I have observed amazing atmospheric lighting above strip malls or subjects outside my genre of nature photography.
Experiencing this bright little section of the rainbow that afternoon is just another lesson for me to never give up, and always look for that rainbow.
The Science Behind the Photograph
The section of the sky around the rainbow packs a lot of beautiful and interesting optical phenomena.
- We only see a segment of a full rainbow,
- There are radial bright and dark rays that cross the rainbow,
- Inside the rainbow, there are alternating green and purple bands that fade towards the center.
Interesting optical effects are called out in this figure.
Why we only see a segment of the full rainbow
In order to see a rainbow we need raindrops, direct sunlight on those raindrops, and that sunlight has to be shining on rain at an angle of 42 degrees from the shadow of our head (the anti-solar point). We only see a fragment of the full rainbow in this photograph. Why is that?Read More
Note how abruptly the rainbow ends at a certain angle!
Since we can see some of the rainbow, any of a rainbow, then we know the photograph was taken at a location where falling rain, illuminated by direct sunlight, was an angle of 42 degrees from my shadow.
The photograph shows an abrupt transition from the bright rainbow to dark clouds. The light-dark terminus of the rainbow is along a radial line, that is perpendicular to the colored arcs in the rainbow. This is a strong hint that clouds blocked sunlight from illuminating raindrops to the right of the rainbow segment we see.
Other reasons we might only see a segment of the full rainbow include rain not falling across an angular field of view of 84 degrees. We need a wide angular region of the sky with raindrops to see the full rainbow.
What are the radial bright and dark rays?
One of the most striking features of this photograph is the bright and dark rays appearing as radial spokes crossing the rainbow. These pillars of light and dark result from sunlight passing through holes in the clouds and cloud shadows. One of nature’s optical illusions is how these rays appear to diverge from the low sun on one horizon and converge on the opposite horizon. Why is that? Do these bright and dark rays bend as they transit from one horizon to the other?Read More
Alternating dark and light rays converge to the horizon opposite the sun. How could that be? If I turned around and looked at the setting sun, I would see sunrays emanating from the sun, diverging out into space, and yet when I look away from the sun, I see rays converging on the horizon. Did the rays bend as they traveled from west to east? What we are seeing is one of nature’s most surprising optical illusions.
The apparent bending of the sun’s rays is a matter of linear perspective. The sun is very far away. Light rays from the sun are parallel by the time they reach the earth. When we look towards the sun, the rays appear to diverge upwards into the sky, and when we look away from the sun, toward the opposite horizon, they seem to converge to a vanishing point. In reality, the sun rays are perfectly parallel.
Consider what you would see standing on railroad tracks. When you look down the tracks in one direction, the parallel rails appear to converge. Look in the other direction, and they also seem to converge. This phenomenon is known as “linear perspective” and is fundamental to how we perceive parallel lines, vanishing points, and the horizon.
What causes these sun rays to form in the first place? Clouds do! Shadows form when sunlight is blocked by clouds. Air molecules, dust, and aerosols in the atmosphere scatter sunlight towards our eye. Cloud shadows reduce the amount of light incident on these small particles and therefore appear dark.
The bright and dark rays are known as crepuscular rays since they are more apparent during the crepuscular parts of the day (just after sunrise and just before sunset). The Latin word “crepuscular” means twilight.
Crepuscular rays are seen looking towards the setting sun over the Jemez Mountains, New Mexico.
The contrast of crepuscular rays increases when we look along the rays, towards the horizons, and look through a longer path length of shadowed air. An increase in scatterers in the air will increase the contrast. This is evident with forest fire smoke.
The contrast of the anti-crepuscular rays is high in front of the rainbow. Raindrops scatter light back to our eye, cloud shadows reduce the amount of sunlight on the rain and therefore show up as a dark ray contrasting the bright rainbow.
Anti-crepuscular rays at sunrise, looking west these parallel rays appear to converge on the horizon, away from the sun.
It is easier to see crepuscular rays, those on the same horizon as the sun, either at sunrise or sunset, then it is to see anti-crepuscular rays on the horizon away from the sun. When you do notice crepuscular rays, turn around and look for anti-crepuscular rays on the opposite horizon.
How come there are “extra” green and purple rainbows inside the main rainbow?
Inside the primary rainbow, you can see alternating bands of greens and violets that gradually fade away. These are known as supernumerary bows and are a result of the wave nature of light.Read More
Light rays that undergo the same deviation angle when refracting and reflecting from a raindrop reach the observer’s eye after traveling different path lengths inside the raindrop. Although the path length can be defined by suing the concept of a light ray (“geometrical optics“) the wave nature of light now comes into play.
Light waves that travel different distances to our eye can be in phase, meaning the crests and troughs of the light wave are aligned. Other pairs of light waves will be out of phase, with the crest of one wave arriving at our eye the same time a trough of another wave arrives. The resulting light we see is the sum over all the rays in a given direction and over all phases. The different waves interfere with each other. Interference is a phenomena associated with waves, including electromagnetic waves (light), sound and water waves.
When multiple light waves arrive in phase, the light is brighter (“constructive interference“) than when the incident light waves are out of phase (“destructive interference“).
Inside the primary arc of the rainbow we can see alternating bright and dark bands, of different colors (usually greens and purples). These color bands are where the light waves that traveled different path lengths in the raindrop interfere in a way that reinforces (“constructive interference“) the brightness (i.e. light waves that are in phase with each other), the dark areas between the bright areas are where “destructive interference” occurs (i.e. the light waves that reach our eye are out of phase with each other).
The size of the raindrops affects supernumerary bows. Smaller raindrops result in wider spacing between the supernumeraries, making them easier to see. The uniformity in the size of the raindrops also affects how well supernumeraries can be seen. The more uniform the size of the raindrops, the stronger the supernumeraries will stand out.
Limited Edition Prints
Santa Fe Supernumerary Rainbow
(60 x 40 in print shown here for scale)
“Santa Fe Supernumary Rainbow” (c) Ed MacKerrow / In Light of Nature. ( 20180818__DS_3433, 8525:5445, 3:2 )