Category Archives: miscellaneous phenomena

Distrail passing through iridescent cloud

Distrail passing through iridescent cloud · Photo: Claudia Hinz

At 6.35 A.M. on June 25, 2015, I noticed a plane passing through a clear part of the sky without leaving any trace (contrail) behind. Then I observed a beautifully irisating foehn cloud, when suddenly a distrail moved into the cloud dissipating it within two minutes.

Distrail is a short word for dissipation trail. It describes streaky cloud holes caused by airplanes. When a plane flies through or directly above a thin cloud layer, the wake vortices mix the dry air around the cloud into it and the cloud droplets evaporate. This effect is even strengthened by the hot exhausts of the plane, and a clear trail forms behind the plane. Often dust particles in the exhausts act as condensation nuclei making the cloud droplets freeze and form ice crystals. As the saturation vapour pressure above ice is lower than it is above water, the adjacent droplets evaporate. The result is then a white streak of ice clouds between two clear streaks.

Amateur pilots report that the dissipation of clouds also works at small airplanes without jet engines. In this case the propellers stir the air making the cloud dissipate.

Author: Claudia Hinz, Fichtelberg (1215m), Erz mountains, Saxony

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Unexpected black drop effect

This phenomenon is well known from the transit of Venus in front of the disc of the sun. This effect appears when the objects are not exactly focused (see: The black drop effect is not an atmospheric phenomenon). On Oct. 13th, 2013 I photographed this effect under unexpected circumstances:
The sun had just set behind the skyline of the Palatinate Hills across the Rhine valley, when a very bright contrail due to forward scattering of sunlight raised from behind the Hills. One of the hills covered the contrail, and the brightness contrast showed the drop phenomenon very nicely: the slope of the hill appears almost vertical where it is intersected by the contrail. Due to the far distance, I used the 13x zoom of the Canon Powershot A510. As the optics of such a small camera is limited, it provided the defocusing needed to show the effect. The sequence show the raising of the contrail during a time lapse of 4 1/2 minutes just after sunset.

If you may ask now: where is the “black drop”?: The “black drop” is somewhat hidden: it is the interface area between the bright contrail and the dark silhouette of the hill, where the “drop effect” raises the skyline showing an almost vertical slope of the hill in front of the contrail. The “drop” is best seen on the second and third frames from the bottom.

Author: Christoph Gerber, Heidelberg

Another article to this topic: The black drop effect is not an atmospheric phenomenon

Noctilucent clouds in February?

On February 20 and 24, 2013, unusual clouds which looked like NLC (noctilucent clouds) were observed by the pilots Terry J. Parker above Birmingham, UK, Nikolay N. Nikolaev and Egor C. above Moscow, Russia (picture at top).

In the AKM forum (AKM = Arbeitskreis Meteore/ Meteor Workshop) there were discussions on what could have been the reason for this unusual phenomenon.

  • Polar Stratospheric Clouds can be excluded as a reason because the stratosphere was too warm at the time the observations were made (12).
  • MAARSY (Middle Atmosphere Alomar Radar System) in Andenes, Norway, recorded strong echoes in the mesosphere. Weak echoes have also been recorded at Kühlungsborn, Germany. Mesospheric winter echoes are common, but up to now there are no special observations recorded which were connected to these echoes.
  • There was a full moon on February 25, so high cirrus clouds illuminated by the moon cannot be excluded as a reason.
  • The most probable explanation for the phenomenon is, however, that the clouds had been caused by the meteor which hit near Cheljabinsk, Russia, on February 15. A recent study by Kathryn Hansen shows that this cloud of smoke travelled around earth in an altitude of about 40 kilometres. This fits with altitude measurements made in Wales, which showed that the unusual clouds had formed at altitudes between 35 and 38 kilometres.

Another article to this Topic:  “Noctilucent clouds in october ?”

Light phenomena in a mosquito net

In August 2013, Ujj Ákos from Bátonyterenye in Hungary noticed light phenomena in a mosquito net for several times. There were phenomena visible (1234) which resembled halos, similar to those in a spider web which Christoph Gerber photographed, but also iridescent colours were visible (12). Unfortunately, it is difficult to see what exactly causes the brightenings, although one can assume that they are caused by forward scattering of light along the different threads. But a mosquito net has no radial structure. So the reason of the phenomena probably lies in the grating of the net causing colourful light diffractions.

Diffraction patterns in fabrics can differ significantly and depend from the material used and its mesh. So crosses and paths of light are formed like those in this picture, or different kinds of coronae which can be seen here.

Wake vortices caused by planes

When air humidity is high, sometimes wake vortices can be observed on the wings of a plane. These are an accessory phenomenon of the ascending force, which needs a certain underpressure to be effective. This underpressure makes the air flow from beneath the wing to its surface for pressurization. As in these vortices there is an area of especially low pressure, the air cools down adiabatically here, often reaching temperatures below the dew point. This makes the water vapour in the air condensate to steam or fog, making the wakes visible.

In the morning of October 8, 2012, Renate Possiel could take a photograph of this phenomenon from the control tower of Munich airport. That day there were wafts of mist with different ranges of sight on the runways. More photographs: 123

Another reason for wake vortices to form is the downward acceleration of the air along the wings when the plane is ascending. At low temperatures and high humidity, also here visible condensation can occur. When a plane passes near the sun, sometimes an iridescence of the wakes can be observed, as showed in this photograph taken by Gabor Metzger.

More often iridescence can be observed on contrails, especially when they are very “young”.  Also these photographs (123) taken by Gabor Metzger.

Another articles to this topic:

Fly Wing Interferencies

On May 14, 2012, Daniel Gerstgrasser photographed beautiful interference colours on the wings of a fly near Zürich. (Camera Canon , 100-mm-macro-lens, f11/125sec, ISO 100). The colours are caused by interferences. The structure of the wings which consist of thin, stacked layers with different refraction indices reflect the light from each boundary where the refraction index changes. So the incoming light is split up into a wave reflected from the front side of the layer and another wave reflected from its rear side. Both waves meet in front of the layer causing the interference.

This is because one wave has to pass through the layer while the other one is already reflected from the front side. So both reflected waves are shifted against each other due to the difference in time they need for these different ways. As the different colours are caused by a difference in light wavelength, it depends from the colour of the light if this shifting causes an amplification or an extinction of the respective colour. If, for example, red is extinguished, the complementary colour, which in this case is green, remains. So the wing shimmer greenish.

Light crossing the layer in a flat angle has to cover a longer distance inside the layer. This causes a different shifting against the wave reflected from the front side of the layer than it is compared to light leaving the layer in a more pointed angle. This is why there appear different colours when looking at the wing from different directions.

Colours in Citric Acid

Lately I experiment a lot with crystal growing and I knew that Citric Acid can refract the light, but when I saw this I was more than surprised. These flat crystals of about 1-2 cm in diameter (the larger ones) were grown between two glass plates, then put in front of a polarized light source and photographed with a polarization filter. It took a few tries to find a proper way to grow them flat enough for this purpose though. More Photos see here and here.

Author: Rolf Kohl, Germany

Spiderweb-Halos

Trying to photograph a spiderweb against the sun to get a completely forward-scattered image of it led me to the discovery of the “spiderweb-halos” (Foto 1). I got a rather dim image of the web against the darker background, but I was struck by a much brighter circle-like structure. This was the moment of the discovery of “spiderweb-halos”.

All you need to get them is a spiderweb between yourself and the sun and a device to block the sun. I used my self-made filter usually used when photographing halos, made of a small piece of black slide film inserted on one end of a ball pen’s tube. It has the right transparency to show also the contour of the sun disc on the photographs (Foto 2). One of the best pictures I get is this (Foto 3).

The spiderweb-halos belong to the general phenomenon called “Circles of light in treetops” (M. Minnaert, Light and Colors in the Outdoors (1993), §29: Foto). As seen on the pictures, the bright forward scattering part of each thread is that one, which is perpendicular to the sun. These segments form an (interrupted) circle between the spiderweb centre and the sun.

Taking several pictures I discovered the dynamics of the spiderweb-halos (Foto 4). The diameter of the circle increases with increasing distance of the sun from the centre of the web (Foto 5), until the halo becomes a dim and open circle segment when the sun is far outside the web (Foto 6). With the sun in the centre of the web, the whole web will be lit up as a “full halo” (not perfectly realized in Foto 2).

An outer circle segment may also be seen, depending on the construction of the web (Foto 4). Another feature of the spiderweb-halos known to halo observers is the “pillar” between the sun and the center of the web. The pillar may vary in appearance from a truly pillar-like structure (Foto 7) to a triangular segment (Foto 8). The pillar may also be seen outside of the sun (if the sun is located near the centre of the web; Foto 9) and extend to the other side of the center of the web (“counter-pillar”, Foto 7).

The full-scale of  “spiderweb-halos”  is shown in this very schematic diagram – it visualizes also why I termed them “halo”: all appearances are known also to the atmospheric halo observer.

Author: Christoph Gerber, Heidelberg, Germany

Spider silk glitter path

While taking a walk, I noticed a field that was covered with fine spider silks. The sun was rather low (about 10°) and made a kind of lower light pillar appear in the silks. When I took a closer look, I could also see concentric circles of light with the sun in their centre.(2)

These circles are caused by the perspective and the angle in which the light strikes the surface of the spider silks. The light gets reflected best to the observer when the reflecting surface is positioned at an angle of 90° to the source of light. A similar effect can be observed when a street lamp shines throug wet branches of a tree.(3)(4)

The “lower light pillar” can be seen better because the sun as the source of light shines vertically down onto the field and all spider silks in this direction reflect the light towards the observer.

Place : Kämpfelbach, Germany
Time : 01 November 2011
DSLR Camera : Canon EOS 450d
Exposure : 1/60 sec, f/22mm, F/10, ISO 100

Author: Michael Großmann, Kämpfelbach, Germany

Noctilucent clouds in october ?

Foto: Rüdiger Manig, Neuhaus, Germany

In the evening of October, 21, 2011 between 18.45 and 19.15 CEST in Czech Republic, Austria and Germany spreads of clouds were observed from the northwestern to southwestern horizon at a maximal altitude of 15-20°, which reminded very strongly to noctilucent clouds (NLC). But NLC is not ineligible as explanation for this season of course and also the cloud structure’s altitude calculated from the sun elevation (-8° to -11°) is ca. 60-70 km is too low for NLC.

All known observations with observed direction of clouds (if noted).
Black Points: Observations without photos
Red points: Observations with photos (meteoros-forum)
Violet points: Animations (123)

The most probable cause could be the exhaust gases of the Soyuz rocket, which was launched in Kourou, French Guayana at 12.30 o’clock, in order to bring the first satellites for the European navigation system Galileo into the earth orbit. During the overflight above Europe a large extent of water, carbon dioxide and nitrogen containing exhaust gases were ejected with a speed of approximately 4km/s and fell downwards unimpeded. At a height of approx. 70km the water vapour condensed into clouds, which were illuminated –  like the NLC – also by the sun, while normal clouds lay long in the shade.

A further possibility of explanation could be a meteor trail. There is one not too much detailed observation, which could refer to a fireball: Report in German and with automatically Bablfish-Translator. But this possible fireball was in this moment, in which would observe the NLC-shaped clouds (18:45 CEST). And as experience teaches a meteor trail needs several hours to develop.

Authors: Landy-Gyebnár Mónika, Claudia Hinz, Wolfgang Hamburg