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

Extremely red rainbow

This red rainbow appeared while the sun was setting and persisted even some minutes after sunset. At that time it was rather dusky already, and the glowing tops of the distant Alps appeared rather unreal. A short time later, a red rainbow appeared, showing an intense red colour which in this intensity I had never seen before. It showed its maximum intensity about 5 minutes after the calculative sunset, but the sun had already sunk behind a mountain some time before. After only a few minutes, rainbow and afterglow faded away simultaneously. The picture is a panorama made of 4 portrait frames with the single frames slightly underexposed, but not processed. The pictures are taken at ISO 800, shutter time 1/40 sec, f/4,5, and a polarizer was used.

At such low sun elevations, all short waved colours of the light are scattered away on the long way through the atmosphere, leaving only the long waved red light behind. This red light reaches the observer´s eyes as alpenglow and as a red rainbow. As, due to their altitude, the clouds (in this case altocumulus in about 3.000 metres) receive sunlight even longer than the ground, in rare cases a rainbow can even be visible after sunset.

Twinned rainbow over Dresden, May 11th, 2012

During the late afternoon of May 11th, 2012, a mild thunderstorm moved from west to east over Dresden. I was located at the University campus (51° 02′ N, 13° 44′ E) south of the city and seemingly at the southern edge of the cloud. At 17:30 CEST I noticed the beginning of rainfall, and at 17:32 the cloud already gave way for the sun again in the west, while the peak intensity of the rainfall was just reached at my place. I could trace the primary rainbow in the glittering of nearby drops at this time, similar to the appearance of halos in close ice crystals which are frequently observed at cold placed during winter. Already at this stage, the primary looked unusual, and the idea of twinning occurred to me. So I rushed through the building to fetch my camera and find a suitable location to take photos. I arrived at my preferential observation window at 17:36. At this time, local rainfall had almost completely ceased, and a beautiful double rainbow display could be seen in the east (Fig. 1, 17:37:44, Pentax K-5 + Zenitar 16 mm).

The primary rainbow showed a rather broad single supernumerary around its top, which could be interpreted as a weak form of twinning. Within the next minute, some cloud (maybe a part of the cumulonimbus that was still above my place, though not producing rain anymore) did cast a localized shadow on a part of the veil of raindrops in the east, and a very distinct twinning of the primary bow near its top became visible (Fig. 2, 17.39.06, Pentax K-5 + Pentax DA 18-55 mm at 18 mm, with boosted contrast and unsharp masking)

The most convincing theory for twinned bows so far is the assumption of a mixture of smaller and larger raindrops within the shower, with the larger ones being more flattened or squeezed in the vertical due to the resistance of air during falling [1]. According to this, the primary rainbow produced by the larger drops is shifted inwards (i.e. towards the antisolar point), especially near the top. However, as calculations show, the secondary rainbow remains almost undisturbed. This behavior of the secondary was exactly what I could record during my observation (Fig. 3, 17:39:12, Pentax K-5 + Pentax DA 18-55 mm at 40 mm, with boosted contrast)

The twinning was visible up to 17:41, when the top of the rainbow faded away due to the growing shadow of larger clouds in the west. The left part remained visible for some more minutes (Fig. 4, 17:43:44, Pentax K-5 + Pentax DA 18-55 mm, single frame extracted from video file). At 17:42, a complete and intense double rainbow could be observed also from a location 5 km north of my place, with a well-developed supernumerary inside the primary but without any signs of twinning [2].

For the image taken at 17:39:06, I performed a detailed analysis of the position of the twinned primary and the un-twinned secondary rainbow. The sun was located at this very moment at 26,9° in elevation and 265,9° in azimuth. To calibrate the photo with respect to focal length, elevation and azimuth of the image center as well as rotation around the optical axis, I took a star field image in the late evening of May 14th, 2012 from the very same place. From the position of two stars, the star field photo can easily be calibrated, which allows to calculate the positions of two landmarks at the horizon. Their positions allow in a second step the calibration of the original rainbow image [3]. Lens distortion was not considered, i.e. perfect rectilinear projection was assumed, since all pictures with the zoom lens were taken with activated real-time distortion compensation (camera feature). The results are very convincing, indicating the high accuracy of this calibration method when carried out carefully. This can be illustrated nicely by an overlay of the actual image with the theoretical positions of rainbows made by spheres (Fig. 5)

Only geometric optics was used here, and from it only the Descartes angles for the monochromatic wavelengths of 600 nm (red), 530 nm (green), and 460 nm (blue). Furthermore, I transformed the image into a sun-centered equirectangular projection, in which the rainbows from spherical drops have to appear as straight horizontal lines, indicated by the marks on both the left and right side of the image (Fig. 6)

The inward shift of the lower branch of the primary is obvious, with no part of it extending into Alexander’s dark band, i.e. beyond the Descartes angle. This is in accordance with the theory for flattened drops, as well as the secondary not showing any measurable shift or distortion and furthermore sticking closely to the Descartes angle all along its visible extension. It should be noted that more exotic splittings of the primary have been observed [4] whose explanation requires a different theoretical approach.

Taking a closer look at the shadow edge near the top of the primary, it seems that coming from the left the “ordinary” rainbow, i.e. the upper branch, is dimmed in the shadow region, but can be traced up to +20° in clock angle. On the other hand, one marks a smooth transition from the supernumerary in the bright region on the left into the inner branch of the twinned bow in the shadow region on the right. It is obvious that this localized shadow which is cast by a smaller cloud segment will prevent a certain set of drops in this very direction from contributing to the primary rainbow, and that the remaining sunlit drops are in the right mixture to give rise to a clear twinned bow. Very likely the drops a few degrees to the left will not exhibit a drastically changed size distribution. However, the drops in the foreground are illuminated there and add to the overall primary brightness, thus covering unfortunately the twinned bow. One can speculate that there might be many twinned bows in nature that are hidden from us due to the contribution of “ordinary” drops in front or behind the interesting region along the rainbow cone.

[1] http://www.atoptics.co.uk/rainbows/twin1.htm
[2] http://www.meteoros.de/php/viewtopic.php?t=9492
[3] http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-50-28-F134
[4] http://atmospherical.blogspot.de/2009/11/twinned-rainbow.html

 

Author: Alexander Haußmann, Dresden, Germany

Cloud bow

On December 23, I observed on Mt. Wendelstein (1838m, Bavarian Alps) my first real cloud bow which formed on the rim of a cumulus – or might be even a cumulonimbus – cloud. Although a snow shower had formed in the centre of the cloud, the bow clearly did not appear in the shower itself but on the outermost rim of the cloud. There, the cloud droplets must have been even big enough to make the bow show faint colours.

When the phenomenon started to appear, there was the left end of a rainbow visible at the rim of the shower, so it might have rained there. But the picture clearly shows how the droplets become rapidly smaller as the bow extends into the cloud. Even its diameter seems to be smaller in its upper part (1 – 2 – 3).

The crossed bows

What kind of rainbow is that? This is no fake this is real! Ok…it’s a little trick with an open window and the right angle to the sun.

The rainbow are produced with a water spray bottle. The right bow is the “real one” and the left bow is the reflected one.

The reflected surface in this example is the window in vertical direction, so the bows looks like a “x”. (2)

Place : Pforzheim, Germany
Time : 18 May 2011
Digital Camera : Panasonic DMC-FZ50
Exposure : 1/200 sec , f/3.6 , ISO 100

Author: Michael Großmann, Kämpfelbach, Germany

A single water drop

A slight-projector and a singel water drop shows a lot of bows. Here you can see the primary, secondary and tertiary bow.

The distance between the water drop an the projection backside (white paper) is 30 mm, waterdrop an light source has an diameter of 2 mm.

Photo taken on 28.07.2011 on my desktop

Author: Michael Großmann, Kämpfelbach, Germany

Discontinuous Rainbow in Front of Ridge

There have already been observed discontinuous rainbows above a ridge for several times. (For example by H. Edens and C. Hinz). Explanations range from an optical illusion via burst raindrops up to the assumption that there are only large flattened raindrops in front of the ridge which reduce the radius.

This is why during my latest observation on June 16, 2011, on Mt. Wendelstein (1838 m), I made the effort to also watching the raindrops. During my observation, there were wind gusts of up to 34 m/sec (122 km/h). In addition, on (my) mountain slope, there were heavy turbulences making the large raindrops come from all directions, even upwards the steep northern slope. These were most deformed of all, some had the shape of vertical ellipses, and some were even almost triangular. Unfortunately, due to the storm I was unable to take valid photographs of the raindrops.

From Physical Review Letters (DOI: 10.1103/PhysRevLett. 101.234501) I learned that raindrops of a diameter of about 1 cm and making 3 rotations per second take a triangular shape. Such kind of rotations must have occurred on my mountain as well as on the neighbouring one where the rainbow appeared.

I am sure that such deformations alter the diameter of the rainbow causing those breaks. Does someone have the occasion to simulate rainbows on such raindrops?

Author: Claudia Hinz, Brannenburg, Germany

Near Infrared rainbow

Since Newton specified rainbow colors in 1672, we have been told that a rainbow has 7 colors : red, orange, yellow, green, blue, indigo and violet. These colors are thought to be a representative of how humans see rainbows everywhere. In fact, a rainbow spans a continuous spectrum of colors (no bands).

If there are rainbow bands in the visible spectrum, then there should be rainbow bands in the rest of the spectrum as well, but we just couldn’t see them directly. A near IR sensitive digital camera and a near infrared pass filter allow the near infrared rainbow to be captured.

A near infrared rainbow lies next to a red band of any rainbow, simply because the near infrared region lies right next to the visible red region of the electromagnetic spectrum.

The visible and near infrared rainbows were captured in the evening of June 18, 2011 at approximately 5:24 PM in Bangkok, Thailand. The rainbow showed up after a quick rain and last for about half an hour

(see time-lapse video : http://www.youtube.com/watch?v=pTz-xVjhHPc).

Camera and filter used :
Camera : FujiFilm IS-1
Near IR Pass Filter : Hoya R72 IR Filter
UV/IR Block Filter : Heliopan Digital UV/Infrared Lichtfilter Slim version

FaceBook Album:
http://www.facebook.com/media/set/?set=a.210017799039730.49215.199991580042352

Author: Pitan Singhasaneh, Bangkok, Thailand

Valley rainbows

When working on a mountain top, one very soon breaks the habit of looking for rainbows only in the sky. Here rainbows can appear at all sun elevations, even when one really does not reckon with them. Last year I could watch rainbows at sun elevations of more than 60° on Mt. Wendelstein. The most beautiful ones appeared when several rain showers passed on May 31, 2010. The maximum sun elevation during this observation was 63.6°.

Photos: 1 – 2 – 3

Later the same day (sun elevation now was “only” 41.8°) i had the rare opportunity to see a part of a rainbow on the left side of the mountain, while at the same time there was a fogbow on the right side, which soon was replaced by a glory. Unfortunately, it was impossible to look from the northeastern part of the mountain at the same time, so I could not see the transition from rainbow to fogbow.

On this day, rainbows appeared 6 times, the last one was a double reddish rainbow over the Inn valley.

Author: Claudia Hinz, Brannenburg, Germany

Double dewbow

On June 25, 2010, Rüdiger Manig observed a double dew bow on unevenly spread morning dew on a leaf in Neuhaus am Rennweg (Thuringia, Germany).

Especially when he slightly defocussed his camera, the bow could be distinguished better.(1-2-3)

Actually, the distance between the two bows was less than 10°, the angle which one could expect in a double dew bow. Maybe, however, that the angle of refraction was significantly reduced by the deformation of the droplets on the leaf.