If the weather conditions allow it, then fascinating plays of light appear around the sun: circles, arcs, even apparently a side sun, a little like on Tatooine. They are called a small circle, or halo. The latter term comes from the Greek and denoted a ring of light around the sun or the moon. These halos are caused by cirrus or cirrostratus clouds at high altitudes when, after a stable high pressure area, a low approaches from the west.
They form in 5 to 13 kilometers from rising water droplets which freeze at this height, ideally with a very slow growth, caused by a leisurely saturation of the air with water vapor. These ice crystals are often very uniform and each have the shape of a hexagonal prism. The difference to the rainbow is that here the light is refracted by the ice crystals – not by the water droplets.
Arc from an angle of 22 °
And in these millions of mini-prisms, the sunlight is refracted at will, although it is brightened at an angle of 22 °. The light beam enters a prism surface and becomes – just like in a hexagonal prism [– ] refracted at an angle of about 22 ° to 46 ° and ejected via the next but one prism surface. The halo becomes perceptible at this angle between the light object such as the sun and the viewer. This is also called the minimum distraction.
Typically, the ring of a halo is thin and light. The inner edge is sharply outlined and reddish in color, as is often the area below. This is because at the minimum of the deflection, most of the light rays reach the viewer. The further the entrance angle is removed from this dimension, the more the light rays are refracted. Therefore, the outer edge is usually milky-diffuse.
Halos also visible around various light sources
Halos can also be seen around the moon. Since the color receptors in the human eye need a certain brightness in order to be able to recognize colors, halos are only perceived in white and gray shades. If there is one fact, Star Trek fans hope JJ Abrams will never find out about it not to give him foolish thoughts: Halos are possible under the conditions around any light source. Even snow-covered slopes can create certain types of halos under certain conditions. The phenomenon is not a short matter and does not depend on the position of the sun: a 22 ° halo can often be seen in the sky for several hours.
A selection of other halo types can be seen in the text box below. With different ice crystal shapes, it is possible that different types of halos occur together. Some halos only occur in connection with others. It is also important to note that such solar phenomena should never be viewed with a direct look at the sun in order to avoid possible eye injuries. Therefore, the sun should always be covered, for example by a traffic sign or light pole.
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