日暈(ヘイロー)
暈(かさ)とは、太陽や月に薄い雲がかかった際にその周囲に光の輪が現れる大気光学現象のことである。特に太陽の周りに現れたものは日暈(ひがさ、にちうん)、月の周りに現れたものは月暈(つきがさ、げつうん)という。虹のようにも見えることから白虹(はっこう、しろにじ)ともいう。
白虹貫日
日暈は別名白虹ともいうが、中国では古代、白虹が太陽を貫くことは、兵乱の兆しとされた。白虹は干戈を、日は天子を表すという。司馬遷 (紀元前145年 - 没年不詳) の『史記』鄒陽列伝に「白虹日を貫けり。太子畏ぢたり」とあり、燕の太子丹(たん)の臣、荊軻(けいか)が始皇帝暗殺を謀った際、白い虹が日輪を貫き、暗殺成功を確信させたが、それでも丹は計画の失敗を恐れたという故事が見られる。他にも「彗星(妖星)の飛来」「太陽が2つ現れる」などが兵乱、大乱の予兆といわれるが、2つの太陽とは「幻日」のことであり、それと同時に観測されることが多い「幻日環」がここでいう白虹のことではないかとする説もある。また、戦前の大阪朝日新聞が報道記事に「白虹日を貫けり」という文言を掲載したために当局より弾圧を受けたという事件が起こった。
Sun Halo
A halo (from Greek ἅλως, halōs;[1] also known as a nimbus, icebow or gloriole) is an optical phenomenon produced by light interacting with ice crystals suspended in the atmosphere, resulting in a wide variety of colored or white rings, arcs and spots in the sky. Many halos are near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the most well known halo types are the circular halo (properly called the 22° halo), light pillars and sun dogs, but there are many more; some of them fairly common, others (extremely) rare.
The ice crystals responsible for halos are typically suspended in cirrus or cirrostratus clouds high (5–10 km, or 3–6 miles) in the upper troposphere, but in cold weather they can also float near the ground, in which case they are referred to as diamond dust. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting light between their faces, sending shafts of light in particular directions.
Atmospheric phenomena such as halos were used as part of weather lore as an empirical means of weather forecasting before meteorology was developed. They often do mean that rain is going to fall within the next 24 hours as the cirrostratus clouds that cause them can signify an approaching frontal system.
Other common optical phenomena involving water droplets rather than ice crystals include the glory and the rainbow.