Category Archives: twilight phenomena

Crepuscular rays from above and below

Yesterday there were observations of spread Crepuscular rays over Germany. The satellite image shows the origin of the long shadows: a powerful squall line over northwest Germany. The length of the shadows is about 400km – this is enormous!

20160623-2115

Near Pforzheim in Baden-Württemberg Michael Großmann observed rays passing from the setting sun to the antisolar point. Rene Winter was in the district Gotha, Thuringia and saw crepuscular rays that were unusual intensively. Laura Kranich in Kiel wasn’t far away from the thunderstorms and had intense Crepuscular rays, too. There were single beams that ran across the entire sky.

Crepuscular rays are rays of sunlight that appear to radiate from the point in the sky where the sun is located. These rays, which stream through gaps in clouds (particularly stratocumulus) or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions. Despite seeming to converge at a point, the rays are in fact near-parallel shafts of sunlight, and their apparent convergence is a perspective effect (similar, for example, to the way that parallel railway lines seem to converge at a point in the distance).

The name comes from their frequent occurrences during twilight hours (those around dawn and dusk), when the contrasts between light and dark are the most obvious. Crepuscular comes from the Latin word “crepusculum”, meaning twilight.

Red Sky Glow by reflected Dawning

In the morning of January 24, 2015, I noticed that the sky was covered with low clouds, except for a small gap right above the southeastern horizon, where the sun would rise about 20 minutes later. So I expected a wonderful dawning, but nothing happened. But at 8.10 a. m., I noticed a strange red light coming from the west. When looking out of the southward window, I saw the western and southwestern sky glowing in a dark red colour.

During the following minutes, the red glow slowly extended eastward and became more and more intense. At last it was so intense that even the ground took this colour, giving that morning a quite eerie mood. In the picture taken in northeasterly direction (2) you can see this reflex on the ground, especially on the gravel path and the pedestrian crossing at the lower left. And the picture also shows that towards the east the low clouds still had their normal dark grey colour. In the southeast, no trace of a normal dawning was visible, but higher in the sky there was also this strange red glow from above. (3) This is vice versa to the normal course of a dawning, where the red colour spreads from east to west.

During the next 5 minutes, the light from above became even brighter and turned into a more orange colour (4). At 8.25 a. m., just before sunrise, a bit of sunlight reached the lower surface of the low cloud layer, but it was by far not as intense as the glow coming from above (5).

Author: Peter Krämer, Bochum, Germany

Crepuscular rays extended to (almost) 180° observed from Mt. Großer Zschirnstein, Elbe sandstone mountains, June 8th, 2014

2014_06_08_2126S_IMGP3399_3402_3405_ Panorama_crop_fil

Each year during the Pentecost holidays I undertake together with some friends a cycling tour to the Elbe sandstone mountains. This is usually a good opportunity to look for atmospheric phenomena, since we are out in the open the whole day. However this year we just had the sun shining from a plain blue sky most of the time. I feared that nothing interesting would happen, but I was wrong: In the evening of June 8th, thunderstorms were active about 200 km or more to the northwest from our location (Großer Zschirnstein, 50° 51′ 23″ N, 14° 10′ 34″ E, 561m). The top parts of these clouds acted as apertures to cast crepuscular rays through the sky shortly after our local sunset. To the south the view from this mountain is fully unobstructed since the lookout point is located right above a 70 m high rock cliff. Our struggle to thrust the bicycles up there was rewarded by the beautiful sight of a bright, rosy coloured beam extending from the twilight sky in the northwest to the rising earth shadow in the southeast and passing just below the waxing moon.

Even with a (full frame) fisheye lens it was hard to capture due to its extension of about 180°, so I decided to do panorama stitching from an image series (21:26 CEST: local solar elevation -1,5°). One should keep in mind that in reality crepuscular rays are straight lines and the curved shape in the photo is just a result of the cylindrical projection. Likewise it would have been possible to distort the horizon and make the crepuscular ray straight. Having a look at a panning video may be the best way to understand the geometry. Some minutes later (21:31 CEST: local solar elevation -2,3°) a second beam had appeared quite prominently above the first one, and even more might be detectable by image processing. Though all of them being parallel straight lines in 3D space, the mind is always tempted to interpret them as fanning beams like the emissions from a lighthouse.

Until 21.40 the rays disappeared almost completely apart from the foremost part in the northwest, which itself became quite bright at that time. Around 21.48 the cumulonimbus clouds themselves became visible for a while. This change in illumination and visibility must be caused by the increasing solar depression below the horizon which leads to more vertically inclined sunbeams, until the sun finally sets at around 52° N / 12° E (where the clouds might have been) in 10 km of altitude as well.

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 ?”

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.

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

Greece Volcanic Twilights

From August 2 to August 16, I was on holiday on the Greek island of Karpathos. Already on the first evening there, on August 2, I was astonished about an intense purple light with crepuscular rays. These purple twilights appeared every evening at about 10 minutes after sunset an were visible for about 10 minutes. This continued over the whole two weeks, except on August 4, when the purple light was only a bit brighter than normal.

The most intense purple twilights occurred on August 9 and 11, when even the water of the Mediterranean Sea turned purple during these twilights.

Between August 9 and August 14, the purple twilight was followed by an intense, dark red glow above the cloudless western horizon, which was visible until about 35 minutes after sunset. Just before sunset, the sunlight illuminated the Kali Limni, the highest mountain of Karpathos island (1215m) in a rose and violet shade, causing a kind of alpenglow which was visible from the beach.

As I learned after my return at home, these unusual twilight effects were caused by volcanic clouds emitted by Mt. Nabro in Eritrea in June and July.

Author: Peter Krämer, Bochum, Germany

Crepuscular rays in colourful twilight

In August 2011 I noticed colourful twilights with purple light and venus belt in France and Germany during some days. These intense sunsets resulted from volcanic aerosols in the atmosphere.

On August 12, 2011 I could take some photos of the sunset with the purple light near the Gorges du Verdon (Provence/France).  Additionally, there were beautiful crepuscular rays in the sky (photo on top).

Back in Germany I could photograph also such fantastic sunsets with crepuscular rays and anticrepuscular rays.

These pictures (12) were taken near Augsburg, Bavaria, Germany on August 18 and 23, 2011.

Author: Daniel Eggert, Augsburg, Germany

Volcanic twilights again

For a few days since 13 Aug. 2011, observers in Germany have noticed colourful twilight phenomena like intense crepuscular rays, and thus were reminded of the volcanic twilights from the Kasatochi and Sarychev. An aerosol layer is presently verifiable in the entire northern hemisphere, indeed. At the moment, measurements from the Meteorological Observatory Hohenpeissenberg (Germany), Evora (Portugal), Mauna Loa (Hawaii), Ukraine and Russia, all record this layer at heights between 12 and 19 km.

Most probably, these volcanic aerosols can be traced back to the Nabro Volcano in Eritrea. Despite having undergone no historically reported eruptions, the Nabro Volcano erupted shortly after local midnight on 13 June 2011, after a series of earthquakes ranging up to magnitude 5.7 in the Eritrea-Ethiopia border region. Its ash plume was observed on satellite images and drifted to the west-northwest along the said border, spanning a width of about 50 km and extending for several hundred kilometers westward in the immediate hours following the onset of the eruption, while reportedly reaching a ceiling near 15 km of altitude. The ash cloud also disrupted air traffic, as United Arab Emirates based flights were cancelled along with Saudi Arabian Airlines flights. Egypt’s Luxor International Airport was placed in a state of emergency for a while.

This aerosol layer seems to have been present since 15 July 2011 as shown by the Lidar measurements from Hohenpeissenberg.

More pictures and plots of the measurements are summarized here (PDF download):

Link to the NASA-Website with further measurements.

Further posts to this topic: Crepuscular rays in colourful twilight and Greece Volcanic Twilights

Support for this documentation on behalf of the Meteorological Observatory Hohenpeissenberg is gratefully acknowledged.

Author: Claudia Hinz, Brannenburg, Germany

Ozone makes twilight wedge blue

Normally, the twilight wedge appears in a rather grey colour. But sometimes, in most cases above an inversion layer, the twilight wedge appears tuquoise-coloured as during this morning twilight on March 2, 2011. The night before had been so clear that the zodiacal light was visible for the naked eye. Towards the sun, above the first light of dawn, the crescent of the moon and Venus made a nice contrast to the blue sky. On the opposite side, there was a definite twilight wedge showing a rarely clear blue colour. This colour is caused by light absorption in the ozone layer.
Above the twilight wedge there was also the venus belt visible. (Photos 1234)

Author: Claudia Hinz, Brannenburg, Germany