Monday, 10 October 2011

Halo (optical phenomenon) - 091011

                                                              Malaysia 091011

A moon ring, also known as a winter halo, is a phenomenon that usually appears in conjunction with a full moon. There appears to be a whitish ring, approximately 10 to 20 times the size of the moon, surrounding the moon and centered on it. It is caused by refraction of the light from the full moon in the ice particles floating in the clouds, as opposed to a rainbow, where light refracts in the water vapor that makes up the clouds. Since this happens most effectively at a certain angle, this ring appears at the bottom of the clouds, and since similar triangles must form between the moon, the refracting surface, and the observation point, the "highlighted" clouds are at approximately the same distance from the moon, creating the image of a ring.

                                                          Padang, Indonesia 021009

A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in hexagonal ice crystals suspended in the atmosphere. As the light beam passes through two sides of the prism forming a 60° angle, the angle of minimum deviation is almost 22° (namely, 21.84° on average; 21.54° for red and 22.37° for blue). This wavelength-dependent variation in refraction causes the inner edge of the circle to be reddish while the outer edge is bluish. A 22° halo may be visible on as many as 100 days per year.

Light passing through the hexagonal ice prisms is deflected twice, which produces deviation angles ranging from 22° to 50°. Lesser deviation results in a brighter halo along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. As no light is refracted at smaller angles than 22° the sky is darker inside the halo. This effect is similar to Alexander's band, which lies between primary and secondary rainbows.

22° halos form when the sky contains millions of variously oriented (poorly correlated) ice crystals. Some of these happen to be aligned perpendicular to the sun's light as viewed by any given observer, which produces the illuminated 22° circle, while other crystals produce the same phenomenon for other observers.
Like other ice halos, 22° halos appear when the sky is covered by thin cirrus clouds containing the ice crystals that cause the phenomenon. Small colourful coronas much nearer the sun produced by water droplets can occasionally be confused with 22° halos.


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