Glories - an Atmospheric Phenomenon
I am surprised that there is no really good explanation of glories around. I surmised that backscattering is probably at the root of the phenomenon - glass beads used on roadsigns (or the tiny glass spheres of volcanic glass found on the moon) will produce a large reflection in the direction of the incident radiation. Water droplets behave in a similar manner - as can be witnessed when looking at dew-covered foliage with the sun, the moon (or even street lights) behind one's head (a phenomenon more properly called heiligenschein apparently). However, the most striking aspect of the glory is the colour effects which are not so easily explicable. I have tried to get a good optical explanation and have collected the following:
Brendan McWilliams in "Weather watch" in The Guardian (Thursday November 7th 1991), wrote:"Glorious views from the airplane
Samuel Taylor Coleridge gives a lyrical description of a woodsman "winding westward up the glen" on a chilly, misty morning. Suddenly he "sees full before him, gliding without tread, an image with a glory round its head".
It was a rare thing in those days to see a glory. Nowadays we have more frequent opportunities - and we do not have to get up at the crack of dawn to do so. A glory can be seen when an airplane flies in sunlight above a layer of cloud consisting of water droplets - as distinct from the much colder and higher clouds composed of ice particles. It is similar to the corona often seen around the moon - a series of concentric coloured rings - but instead of being centred around the moon, the glory appears around the shadow of the aircraft projected on to the layer of cloud below.
The glory and the shadow are distinct phenomena, but both occur in the same place, at what is called the anti-solar point. They are centred around a point which lies on an extension of the line joining the sun and the aeroplane. If you want to see a glory, you must sit on the side of the aircraft which will be away from the sun.
The corona and the glory are both caused by a process caused diffraction: in essence, the water droplets of the cloud interfere with the direct progress of the waves of sunlight and split it into its constituent colours - the familiar colours of the spectrum. The light which is diffracted to give the glory seen from an aeroplane, is sunlight which has already been reflected inside the droplets of the cloud in such a way that it returns along its original path.
Glories vary considerably in size, depending on the radius of the water droplets of the cloud in which they have their origin. As an aircraft proceeds along its track, a 'glory' may vary in size, corresponding to the changing composition of the layer of cloud below. The smaller the water droplets in the cloud, the larger the diameter of the glory."
Another explanation comes from the course notes of Understanding-Weather taught by Ali Tokay at Saint Louis University, St. Louis, MO; see the notes referring to Chapter 8 of Meteorology by Joseph M. Moran and Michael D. Morgan 5th ed.:"Glory, which is a combination of two refraction and one internal reflection, differs from a primary rainbow such that sunlight strikes water droplets (with diameters less than 50 mm) in clouds and is refracted back in the same direction as it enters the drop, resulting from diffraction of sunlight by cloud droplets of uniform size.Glory appears as color rings around the shadow of the aircraft. The appearance of color in glories depends on which color reaches our eyes from each droplet and what angle each color is reflected from the droplets."
The '50mm droplets' could be a mistake - that's about the size of a snooker ball.
Philip Plait at his 'Bad Astronomy' site says:".... It wasn't a rainbow. It's called a "glory", and is formed in a somewhat different way than rainbows. They both are due to water droplets suspended in the air, but the glory is not due to the simple refraction of light like a rainbow; the light is also "back-scattered". Light from the Sun is usually scattered in the air (which is why the sky appears blue). However, under certain conditions, that light may be scattered preferentially in one direction. It's like a pinball moving through a pinball machine; the ball gets scattered in lots of directions when it hits the bumpers, but since the board is tilted, that scattering usually is downward. A glory has light scattered back towards you, and so it makes the area where the glory appears look brighter than the surrounding area. Even better, the shadow of the airplane usually appears in the center of the glory, since you are looking in the opposite direction of the Sun. All in all, this is a beautiful and astonishing sight, and the next time you're in an airplane, keep your eyes open for it."
The Australian Ultralight Federation has an Aviation meteorology section; their 12.2.3 Cloud droplet effects notes say:"The corona is the diffraction pattern seen in cloud droplets when looking towards the sun, the glory is the diffraction pattern seen in cloud or fog droplets when looking toward the antisolar point. (A glory is the circle of light or aureole around the depiction of the head of a saint etc.) When flying in sunlight over a cloud layer the coloured rings of glory may be seen around the antisolar point, i.e. around the aircraft shadow if it is not diffused. The antisolar point is that of the observer so the luminous coloured halos are centred on the position of the observerís head shadow. As in other diffraction rings the blue halo is on the inside and the red on the outside."
In the Glories section of a Polarization site we read:"The Specter of the Brocken was a rather mysterious phenomena. Somebody would laboriously climb a mountain and break through the clouds into the bright sunlight. Then he would turn around and be confronted by a giant with multi-colored rings around his head. The spectacle was named the specter of the Brocken after the peak in the Harz Mountains of Germany, where it was often seen. The giant figure was the shadow of the climber cast over the fog, who often would exaggerate its size by miscalculating its distance. But the colored rings around the head, the Glory, proved much more difficult to explain. Nowadays the glory is very often seen from airplanes, surrounding its shadow on the clouds below (however, if the airplane is high above the clouds, only the glory remains).The physical origin of the glory was an optical riddle until the seventies, when it was calculated correctly by scattering theory. The small water drops of clouds and fog produce it; however, no intuitive physical explanation is available. In part the glory comes from light guided by the interface of water and air (surface waves) and in part from light that undergoes ten (!) internal reflections (a tenth rainbow?). If you look at a single drop, you will see a ring of light shining on its periphery.The polarization of the glory is extremely unusual. The colored rings are radially polarized, contrary to a rainbow. On the other hand, the white region close to the center is tangentially polarized. With a linear polarizer a distinctive pattern appears, formed by sections of the colored rings plus dark triangles in the center region pointing towards the center.It is worth to look for the glory when travelling by plane as it is quite common. In fact, at one time some pilots used it to find the position of the sun when it was behind them and there was no visible shadow ("shooting the glory"). Interestingly, the pilot will see the glory centered on the front of the aircraft shadow (if visible), while a passenger on the last seat will see it centered towards the tail of the shadow. Many times you will see a long narrow shadow starting at the glory and going backwards. It is the shadow of the contrail, the vapor trail left by the jet turbines."
Other Sites with Information about Glories
- Glories and the Brocken Spectre. This site has photo of the Spectre of the Brocken and also a free downloadable program to simulate the scattering processes in glories and rainbows.
- About Rainbows
- Rainbow Bridge
- Optics Course
- Specter of the Brocken
- Can cirrus clouds produce glories? - pdf file.
My pictures show that the angular size of the glory is not fixed (if it were produced as a result of simple refraction then its angular size would be fixed) - if scattering is involved its size would depend on the size of the droplets - this, indeed seems to be the case. The Rainbow Bridge link above goes into the diffractive aspects of rainbows which clearly apply to the formation of glories.
|Electronics Index||Feedback Form||Go back to glory page|