Sunday, May 15, 2011

Thursday, April 21, 2011

High order bows

A rainbow is a product of millions of falling raindrops interacting with sunlight. A single reflection form the primary bow, a double reflection forms the secondary bow. However, under ideal conditions there can be many more orders of reflection. As shown above, five, six and even ten internal reflections can be observed. Moreover, it's theoretically possible to detect twenty internal reflections, but the problem is to produce a perfectly spherical water droplet. The drops I used for this experiment were formed artificially. The light source is a 5 mW green laser pointer. Note that the bright spot at left center is the laser illuminated water drop.

The third and fourth order reflections aren't shown here because they, along with the seventh and eighth order reflections, are positioned on the other side of the picture in the direction of the light source. The primary and secondary bows will be viewed in the direction you're facing opposite the sun The fifth, sixth, ninth, and tenth order reflections are also in this direction. However, the third and fourth (as well as the seventh and eighth) order reflections can't be seen because they're behind you.

Under exceptional atmospheric conditions it may be feasible to see the third and fourth order bows if you're facing the sun, but they're quite faint. A third order bow, for instance, is one quarter as bright as a primary bow. A fifth order rainbow is only about one tenth as intense as the primary bow.

If you need more information about the experiments with high order bows, you can read this pdf.

Nikon D40X, focal length 18mm, 100 ISO, 2,5 sec. at f/6,3

Author: Michael Großmann, Kämpfelbach, Germany

Tuesday, November 23, 2010

Rays across the sky

The photo features an array of anticrepuscular-rays as observed in Kämpfelbach near Karsruhe, Germany on July 31, 2010. I will never forget this sight. Sunset was fast approaching, and I first noticed faint crepuscular-rays above the western horizon. Just after sundown, the rays could be seen stretching across the sky from west to east. On this photo montage, east is at left center and west at far right. This display lasted for about ten minutes. To add to the show, the rosy glow of Earth's rising shadow (belt of venus) and the shadow band itself were visible just above the eastern horizon (left center). These anticrepuscular and crepuscular rays were cast by clouds below the western horizon. Viewing perspective makes the rays seem to converge toward the horizon; though, they're actually parallel.

Photo details: Nikon D40x camera; 16 pictures in vertical-order; focus length 18 mm; F/3.5; 1/60 second exposure time; ISO 100.


The photo showing a classic cloud-to-ground lightning strike and a cloud-to-cloud bolt as well was taken at Kämpfelbach near Karlsruhe (southwest of germany) on August 22, 2010. Temperatures reached the mid 80s F (25-28°C) here by late afternoon shortly before a weather front and accompanying thunderstorm blasted through. The lightning was quite intense with this storm. I estimated 10-15 flashes per minute. Cloud-to-ground electrical discharges are typically observed when the base of the cloud is negatively charged and the ground is positively charged. On the other hand, cloud-to-cloud (inter-cloud and intra-cloud) lightning occurs most often when the top of the cloud is positively charged and the base negatively charged.

Photo details: Nikon D40x camera; F/9; 1/6 second exposure time; 18mm focus length; 100 ISO.

Dew bow

The photo above showing a sprightly dew bow was captured in a moist field crop at Kämpfelbach Germany on the night October 22, 2010. Since the photo was taken at night, the illuminating source is the almost full moon which is directly opposite of the dew bow at the anti-lunar point. The mechanics of a dew bow are similar to that of a rainbow. Moonlight is refracted and reflected within the dew drops. The city lights in the background are Karlsruhe.

Photo details: Canon EOS 450D camera; F/4; focal length 8 mm; ISO 100; exposure time 30 seconds; 3 photos stitched together.

Saturday, July 10, 2010

Discontinuous rainbow

This is an older observation (from last year). I made it at the Langmuir Laboratory for Atmospheric Research on a mountaintop in central New Mexico, USA at about 10,500 feet altitude above MSL.

The photos look to the east-northeast. The rainbow occurs in a storm that is receding and drifting to the east and has just passed Sawmill Canyon in the foreground. The mountain ridge on the photo is called Timber Ridge, and most of the heavier rainfall is on the other side of that. The much finer, mist-like droplets near the trailing end of the storm are still falling in the canyon and create a rainbow that has a smaller radius and is a little wider than an 'ordinary' bow that occurs in larger raindrops. The effect is very obvious but requires a fairly specific landscape setting to be seen.

The photos were taken on July 27, 2009 using a Nikon D700 camera. Times below are local time (MDT).

Photo 1: The effect is just beginning to occur. 18:37:50 pm, 36 mm focal length, ISO 800, 1/160 sec at f/6.3.

Photo 2: Effect strongest. 18:43:17 pm, 38 mm focal length, 800 ISO, 1/250 sec at f/8.

Photo 3: The rainbow in canyon is disappearing and still shows a discontinuity. Also note that the 'foot' of the rainbow beyond the canyon is not following the circle but appears to kink - i.e. the radius is getting larger at lower altitude - maybe due to drops coalescing and increasing in size as they fall? 18:45:11 pm, 48 mm focal length, 200 ISO, 1/125 sec at f/5.6.

All three photos have not been cropped, modified or enhanced in any way.

Wednesday, May 19, 2010

Bird Feather Iridescence

While taking a walk through the surroundings of my home on February 20, 2010, I took the most of the nice weather by taking some last winter photographs. At 10:09:27 CET, a small covey of about 15 siskins (Carduelis spinus) flew off an alder in front of me and passed me to the right. Seen from my position, they directly passed in front of the sun. I took some photographs with my Sony DSLR-A 700 and a Minolta lens 4/300 mm. The exposure time was 1/8000 second at an aperture of 32 and ISO 200.

Further settings of the camera were: Programme, serial photographs, automatic white balancing, and integral measurement stressed on the centre of the picture. In the original photograph, the sun is almost at the centre of the photograph. The precedent image of the series was exposed for about 1/4000 second at an aperture of 16.

That picture is brighter (a small part of the sun can be seen at the right rim of the photograph!) and the iridescence in the feathers looks rather faint.

Author: Rene Winter, Eschenbergen, Germany

Monday, April 19, 2010

Influences of the Mt. Eyjafjallajökull eruption on the atmosphere

In the morning of April 11, Mt. Eyjafjallajökull, a volcano which is covered by a glacier, erupted in the southwest of Iceland. Its cloud of ashes rises up to altitudes of 10 – 12 kms and has been shifted towards Central Europe by a northerly airstream since Thursday (animation).

The ash particles are slowly sinking downwards in the air, obstructing aviation in many places. In the atmosphere they dim the light (photos C. Hinz 1-2-3) and make Bishop´s Ring visible (photo P. Krämer), which is caused by light refraction on the aerosoles.

In high levels of the atmosphere, the particles act as additional nuclei for condensation, on which humidity (which under normal circumstances is not sufficient for cloud formation) freezes and forms ice crystals generating so-called “Invisible Cirrus Clouds”. Size and/or density of the ice crystals is in most cases not high enough to make the clouds visible, but their existence can be proved by the formation of faint halos such as sun pillars (photo Ina Rendtel), sundogs (photo Reinhard Nitze), or the 22°-halo (photo Brigitte Rauch).

There are still doubts regarding the appearance of the colourful twilight effects known from the eruption of Mt. Sarychev. Measurements with a Lidar effected by the Hohenpeissenberg Meteorological Observatory have shown that most of the aerosoles are at altitudes between 3.000 and 7.000 meters. A heavy rainshower should be enough to wash them out of the atmosphere and make the air clean again. An elevated concentration of sulphuric acid, which after the eruption of Mt. Sarychev formed several layers at different altitudes and caused beautiful purple light and afterglow effects, has not been measured at all. Probably the SO2 ejected by Mt. Eyjafjallajökull is chemically combined to water at the moment when the ash cloud is formed. The explosions, however, are generated by the contact of lava with ice, and every time a part of the glacier falls into the lava, there is plenty of water provided for such a reaction.

Authors: Claudia Hinz, Peter Krämer and Wolfgang Hamburg

Friday, April 16, 2010

Double Rainbow and reflected Rainbow

When I wanted to go to work in the morning of February 24, 2010, I noticed a colourful rainbow forming during a short local rainshower. At first the weather had been fine that morning, and ist was almost calm, but then some rain clouds came up from the southwest and started to cover the sky. The sun was shining brightly at that hour (about one hour after sunrise) and made the rainbow shine in especially bright colours. So I hurried to get my camera and started to make some photographs. While I was taking pictures, I was astonished to see a third bow forming, which intersected with the secondary bow. At the first moment I thought that there was something wrong with my eyes, but I could see it also when I looked at the pictures already taken on the camera display.

I was excited, although I could not make head or tail of it – I had never seen such a thing before. Later, as I searched the world wide web for an explanation, I learned that the phenomenon must have been caused by a reflection. But the reason for the phenomenon was still not clear, as there was nothing between me and the rainbow that could have reflected any light.
Only much later I could solve the mystery: The reflection was caused by the Hallwilersee (Lake Hallwil), which was at about 3 kms behind the position where I had taken the pictures that day. The lake cannot be seen from the place where I saw the rainbow, as there is a hill between the lake and that place. So I did not take this possibility into consideration at first. The position and elevation of the sun, my position towards the lake and the distance from the lake fitted perfectly that morning to form this rare phenomenon for a few moments.

Author: Matthias Frei, Dürrenäsch, Canton Aargau, Switzerland

Friday, April 02, 2010

Bishop's ring

Last Sunday, March 28, I went up on the Magdalena Mountains (in central New Mexico) to pick up two instruments from the laboratory there, and saw what would qualify as Bishop's ring around the sun for most part of the day. The mountain ridge lies at an altitude of about 10,500 feet above sea level. The sky was very clear and dry, and there was light wind at the time.

One of the photos I took is shown to the right. It was taken with a Nikon D700 with a 24-70/2.8 AF-S lens set at 24mm focal length and f/13 aperture.

To the eye the ring had a pronounced blue aureole with brown outer ring. I think the altitude of the observation rules out low-altitude aerosols being responsible. The central blueish aureole was relatively small; it was only a couple degrees in diameter. I estimate the radius of the outer brown ring to have been about 10 to 15 degrees.

Although the atmosphere was stable and quiet at the time I saw the ring, the southwest of the USA including New Mexico has had very strong winds and dust storms over the past week. In general, the spring months see many strong windstorms in this area. I believe the ring was caused by fine dust in the upper atmosphere and not from volcanic activity somewhere.