Translations of Encyclopedia about Geology

Sometimes, when we watch weather, we may see in the sky phenomena, which are incredibly colourful as well as geometrically formed. These phenomena are caused by sunlight. When the sunlight passes through the atmosphere, it collides with air molecules, ice crystals, or water droplets. The light refracts in these atmospheric particles, producing reflection, mirroring, or bending, which we admire. We perceive these phenomena as a rainbow, mirage, aurora borealis, or aureoles.

Another, but completely different kind of phenomena in the sky, are comets. They appear from "nowhere" and are visible, sometimes for weeks, with their characteristic long tail in the night sky. Then, they disappear again into the far reaches of the universe.

Rainbow is one of the most common optical phenomena in the atmosphere. It is produced by reflection and refraction of the rays of sun in drops of water. When a ray of sun falls on a drop of water, upon entrance it is first refracted, then reflected on its surface. This occurs at a variety of angles.

Not all wavelengths of the sunlight refract with the same intensity. Blue and purple portions refract more than red ones. Thus the light is separated into spectral colours and every colour exits the drop of water at a different angle, which means that blue tones exit at another angle than red tones. This produces a conical beam around each drop of water.

This is perceived by an observer as a rainbow of bright colours in purple, blue, and red tones. Red colours are produced at an angle of 42.2 degrees, while purple colours are produced at a 40.4 degree-angle. The intensity of the colour depends on the size of the drop of water. Very small drops of water produce transitional colours, which translate into orange and pink shades. From the point of view of the observer, only a part of this circular formation is visible. If we were in an airplane, assuming a corresponding position of the sun, we might see the entire circle.

Mirage is a phenomenon generated by refraction and reflection of the light in layers of air of different density. Many thirsty desert travellers were deceived by a mirage and believed there was a lake or a river nearby.

When a ray of sun passing through the atmosphere, it gradually enters increasingly dense layers of air. This ray is bent, or deflected, whenever it enters each layer. This process is called refraction. The observer perceives these rays as rectilinear. Under extreme heating of the ground, distant objects move downwards. Thus, as a result of this optical shift, blue sky may appear as water on an asphalt surface, disappearing when the distance gets smaller. In a desert, even trees or buildings, which are behind the horizon, may appear as very close.

Aurora borealis is a light phenomenon occurring in the upper atmosphere and is more common in the polar regions. In the northern hemisphere, it is called also northern lights, and in the southern hemisphere they call it southern lights. Aurora borealis is produced by charged particles of the solar wind, which are captured in the magnetic field of the earth. Protons and electrons bombard at a tremendous speed the molecules of oxygen and nitrogen in the upper atmosphere. They expel electrons from these molecules and enrich them with energy. As the molecules revert to their original state, they emit light. The aurora borealis occurs often in an altitude of approximately 100 kilometres. The region, where this phenomenon is most common, is elliptically 23 degrees from both poles.

This phenomenon occurs quite frequently in combination with the cirrus clouds, when a great number of homogeneous ice crystals is available. The phenomenon forms especially on hexagonal plates or columns of the ice crystals.

Another phenomenon involving light, aureole, is not formed by refraction as is the case of a halo or a rainbow, but rather by bending of the rays of sun, which produces geometrical shapes around the moon or the sun. When the waves of light meet drops of water in a cloud, they are bent by variable force. This produces a colourful wreath of light around the sun or the moon. Its radius depends on the size of the drops of water in the cloud. This wreath is called aureole

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