Halo (optical phenomenon)
Halo (singular of Halo; plural halos or halons) is an umbrella term for lighting effects of atmospheric optics, resulting in ice crystals by reflection and refraction of light.
Depending on the size and orientation of the ice crystals and the angle at which light hits the crystals occur at different points of the sky sometimes whitish, partly colored circles, arches, columns, or light stains.
Etymology
The word " Halo " comes originally from the Greek ( ἅλως ) ie halos, which marked a light ring around the sun or moon ( " Halo " ), or the sun or moon disc itself; other meanings of the word were "Tenne " / " threshing floor " and "slice".
The Greek word was adopted into Latin as halos, and its accusative halo came into German and other languages such as English.
External conditions
This halos may arise which ice crystals have grown as regularly as possible and be clear and transparent. They usually form in large height of 8 to 10 km and their presence is indicated by cirrus clouds. You can, however, form in winter in polar snow ( "diamond dust" ), ice fog or near snow cannons. The regularity of the ice crystals is caused by the slow growth of the crystals as possible, which requires a slow as possible saturation of air with water vapor.
Water crystallizes in the hexagonal crystal system. Thin hexagonal platelets and small hexagonal columns are the ice crystal, while the most prevalent and are mainly responsible for the formation of halos. Small ice crystals of a few tenths of a millimeter long can float in the air, taking for no preferred orientation in the air one. However, if the crystals slowly bigger, they have a correspondingly greater rate of descent and take a stable position, caused by the symmetric vortex on the side facing away from the direction of fall. This is normally only possible with a vertical axis of symmetry, and therefore the crystals by their shape while falling have a maximum drag. Thus, in calm air hexagonal platelets lie horizontally, as well as the longitudinal axis of the columns.
The sunlight is refracted in penetrating such ice crystals, and occurs in response to the orientation of the crystals and the incident angle of the light according to (multistage ) reflection in the interior of the crystals again. At the outlet, it is refracted a second time. Refraction is responsible for the splitting of the visible colors of light. The direct reflection of the light on the external faces plays a minor role in halos.
Even the moon can be observed halo effects. However, the human eye is at low light intensity barely able to perceive colors, so the weaker Moon halos appear white. Halos can be observed under the above conditions to almost any stronger light source.
In addition, can form ice crystals that cause certain types of halos Also on snowy surfaces.
Types of halos
The picture above shows some of the common halo effects and where to find them in the sky.
Are any various forms of ice crystals are present, also different halo effects can occur together. Halos are quite commonly seen in Central Europe, even more common than rainbows. Unfortunately, they are not as colorful as this, and most are towards the sun, which makes them less conspicuous and are easily outshone by the sunlight.
In addition to the above types, there are some less common halo species, including the sub Sundog (s), the Infralateralbogen, Wegener's counter solar arc, Trickers Against Sun arc, the Parry arc, the solar arc, the 9 ° ring, the Moilanenbogen and the counter sun.
22 ° halo, sundogs and horizontal circle (on Matschwitz im Montafon )
Halo deposited by ice crystals on frozen lake, Sigtuna (Sweden)
Halo above Lenzerheide ( Switzerland ) on December 19, 2012
22 ° -halo from the Brocken in the Harz Mountains on December 28, 2012
Halo with sundogs at Echzell / Hesse on August 12, 2012
22 ° lunar halo 23 October 2010, seen from Graz, Austria from
Upper tangent arc and convex Parry arc form just before sunset, a " Double V "
22 ° - and 46 ° halo, halo at Altenberg on January 25, 2014
Physical principles of the 22 ° - ring
Ice crystals crystallize in the hexagonal crystal system. Light that passes through these crystals, as appropriate, as broken, as it would run through a hexagonal prism. Light beams passing through the two surfaces of the ice-crystals, which are tilted by 60 ° to each other are refracted at an angle of about 22 ° to 46 °. At that exact angle between the primary luminous object and viewer of the halo is perceptible. He is, like the rainbow and other diffraction effects, both depending on the position of the luminous object and the viewer.
Visible light has the hexagonal prism minimum deviation between 21.7 ° (red, 656 nm) and 22.5 ° ( violet, 400 nm). No visible light is broken into smaller angles, so that the impression of an empty space between object light and halo is formed. Most of the light beams reach the observer are refracted at angles close to the minimum of the deviation, whereby the perception of a bright inner edge is formed. The inlet and outlet angles are not linearly interconnected. With each degree of the angle of entry is from the optimum, the light is refracted more. Why the halo fades out.
Due to the different refraction of the spectral colors of the inner edge of a 22 ° - ring shimmers often reddish. Nebensonnen arise in the same way.
Physical principles of the 46 ° - ring
Halo this type arises when the light rays are refracted along two surfaces of the hexagonal prism which are perpendicular to each other. This is the case, when a light beam passes through a side surface and the top or bottom of the crystal. The minimum deviation in this beam is 46 °, which is why the ring is exactly where the brightest.
The light rays must meet at a narrow angle on the crystals so that they are broken accordingly, otherwise they are reflected in directions away from the observer. For this reason they appear weaker. In addition, the light is more dispersed, so that the Halo is colorful.
Zirkumzenitalbögen arise in the same manner.