用户:Dalek sg/Circumzenithal arc
环天顶弧是一种光学现象，外观与彩虹相似；
generally in cirrus clouds, rather than from raindrops. It forms no more than one-quarter of a circle centered on the zenith and on the same side as the sun. Its colors are from blue on the inside to red on the outside of the arc. It is one of the brightest and most colorful halos. Its colors are purer than those of the rainbow because there is much less color overlap in its formation.
The 环天顶弧 has been called "a smile in the sky". It is fairly common, but is rarely noticed so far overhead. The first impression is that of an upside-down rainbow.
The light that forms the CZA enters an ice crystal through its flat top face, and exits through a side prism face. The refraction of almost parallel sunlight through what is essentially a 90-degree prism accounts for the wide color separation and the purity of color. The CZA can only form when the sun is at an altitude lower than 32.2°. The CZA is brightest when the sun is at 22° above the horizon, which causes sunlight to enter and exit the crystals at the minimum deviation angle; then it is also about 22° in radius, 3° in width. The CZA radius varies between 32.2° and 0° depending on the solar altitude. Towards either of the extremes it is vanishingly faint. When the sun is above 32.2°, light exits the crystals through the bottom face instead, to contribute to the almost colorless parhelic circle.
References
编辑- David K. Lynch and William Livingston. Color and Light in Nature. 2nd ed, 2004 printing.
- Les Cowley. 环天顶弧. Atmospheric optics. [2007-04-23].
- Les Cowley. CZA - Effect of solar altitude. Atmospheric Optics. [2007-04-23].
See also
编辑