Glories and cloudbows observed during short-distance flights

On Sept 25^th and 27^th, 2014, I was traveling by plane from Dresden to Brussels and back, with stops at Frankfurt and Munich, respectively. As usual, I booked window seats to study sky phenomena. The sunward side was not very interesting, since these short-distance flights are carried out at heights below the cirrus clouds and therefore no sub-horizon halos can be observed (at least in autumn). On Sept 25^th only a single 22° halo appeared in the cirrus clouds above the plane, whereas on Sept 27^th ice crystal clouds seemed to be fully absent.

Accordingly, the viewing direction towards the antisolar point proved to be much more interesting. As most of the Atmospheric Optics enthusiasts I had seen glories and cloudbows before (especially when traveling to the Light&Color meetings in the US) but this time the conditions seemed to be especially favorable. I could observe an an almost textbook-like development of both phenomena right after piercing through an Altocumulus layer after the take off from Dresden (Sept 25^th, 11:13 CEST):

From Debye series simulations (intensity sum of the p = 0 to p = 11 terms in order to prevent artifacts from the small-scale inter-p-interferences as present in the Mie results) a mean drop radius of about 8 µm with 0.5 µm standard deviation can be estimated (assuming a Gaussian drop size distribution):

This simulation was calculated for the original lens projection with added ad-hoc gray background. It is also available as a fisheye view centered on the antisolar point without background [1], together with the corresponding simulation for monodisperse drops (no spread in size) of 8 µm in radius [2].

Unsharp masking and saturation increase processing of the photograph reveals that the sequence of supernumeraries can be traced until they merge with the glory rings:

Over the next minute I mounted the fisheye lens to my camera in order to record a broader view. Unfortunately, some of the outer glory rings and inner supernumeraries had already vanished, indicating an increase in the drop size spread:

Note the smaller angular size of the plane’s shadow as the distance to the Ac layer had further increased. A well fitting simulation to this photo can be calculated by assuming again a mean drop radius of 8 µm and setting the standard deviation now to 1 µm:

For comparison, the fisheye simulation centered on the antisolar point was calculated for the 1 µm drop size spread as well [3]. Furthermore, I recorded a video sequence showing the movement of both glory and cloudbow across the uniform Ac layer (11:15, [4]). When later the edge of the Ac field was reached, the glory showed an appreciable degree of distortion (11:18 CEST [5], processed version [6]).

On Sept 27^th, not a uniform but a fractured Ac layer was present after the take off from Brussels. Nonetheless the glory appeared circular (12:34 CEST [7], processed version [8], video at 12:37 CEST [9]), with the exception of occasional larger disturbances in the layer (12:34 CEST [10]). The cloudbow was not as prominent as two days earlier. During the later part of the flight only occasional Cumulus clouds were present, which did not allow for further glory observations until the plane started descending when approaching Munich. At this point the angular size of the clouds became large enough again to act as suitable canvas for the glory (13:14 CEST [11] [12]). During the final passage through a Cu cloud I recorded a further video (13:15 CEST [13]). Remarkably, the angular size of the plane’s shadow varies rapidly (indicating the distance to the drops) whereas the the angular size of the glory remains rather stable (indicating the drop radius).

Photos and videos were taken with a Pentax K-5 camera equipped with either a Pentax 10-17 mm fisheye or Pentax-DA 18-55 mm standard zoom lens. A gallery view of my photos can be seen here [14].

Alexander Haußmann

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).

Effects in cloud bows caused by perspective

In the morning of December 12, 2008, I coud observe a cloud bow on a stratocumulus layer, which was kind of perspectively cracked. Due to the ruggedness of the cloud surface it seemed as if there was a deep horizontal notch on the left side of the cloud bow.

But also this moonlight cloud bow, taken on September 9, 2008, seems to have vertical indentations and also an elliptical shape caused by the horizontal projection upon an uneven surface.

Günther Können and I have written also an article to this topic.

Author: Claudia Hinz, Brannenburg, Germany

Cloud droplet phenomena

This image shows how light scattering by small cloud droplets produces multiple effects that are actually all part of the same phenomena. The scene was taken by Leigh Hilbert in Washington State in January ’06. The shadow of the descending aircraft is surrounded by a bright glory (1, 2) centred just behind the wing where Leigh was seated. Much further from the aircraft shadow is a circular cloud bow (1, 2), a form of fogbow (1, 2), produced also by scattering by cloud water droplets. The classical light paths producing it are those of the rainbow (1,2,3) but diffraction by the small droplets produces something much broader and almost lacking in colour. Inside the main cloudbow is a supernumerary arc that, characteristically for cloudbows and fogbows, has more colour saturation than the primary. The more distant clouds at the image top have produced a narrower cloudbow indicating that their droplets were larger.

Posted by Les Cowley