Halo - Umbrella

A ring of light with a radius of 22 degrees appears around the sun or moon. The ring of light that appears around the sun is slightly colored, with the inside (the sun's side) being a light reddish brown. The ring of light that appears around the moon is white. It is formed when the light of the sun (or moon) enters, refracts, and is reflected by ice crystals floating in the cirrostratus clouds. It usually has a radius of 22 degrees. This is called an inner halo. A ring of light with a radius of 46 degrees also sometimes appears. This is an outer halo. The color arrangement of the inner and outer halos is the same, with the outside being blue and the inside being red. In addition to rings of light, arcs, balls, and pillars of light, as described below, are formed by the refraction and reflection of ice crystals, and these are collectively called halo phenomena. A white ball of light sometimes appears on the horizon passing through the sun, either on top of the inner halo or slightly outside it. This is a sun dog. An arc that passes through the sun dog and reaches the inner halo at an angle can sometimes appear; this is called a Lowitz arc. An arc of light that appears tangent to the highest (or lowest) point of the inner or outer halo is called an upper tangent arc (or lower tangent arc). An arc of light that appears near the zenith is called a zenith arc. The color arrangement of the zenith arc is red on the outside and violet on the inside, and it only appears when the sun's altitude is low, below 32 degrees, and can look like a beautiful rainbow. Sometimes a pillar of light rises vertically through the sun. This is called a solar pillar, and it tends to appear at sunset or sunrise. In extremely cold areas, the halo phenomenon can also appear when there is ice fog.

There is a weather proverb that says that when a halo appears, the weather will get worse. Considering that haloes appear in cirrostratus clouds, and that cirrostratus clouds tend to form in front of low pressure systems, this proverb is sometimes correct. However, it can also be wrong depending on where the low pressure system passes.

[Masaji Ota]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

太陽や月の周りに現れる半径22度の光の輪。太陽の周りにできる光の輪はやや色がついていて、内側（太陽の側）が薄い赤褐色となる。月の周りに出る光の輪は白色である。巻層雲の中に浮かぶ氷晶に太陽（または月）の光が入射し、屈折、反射してできる。普通は半径22度の大きさである。これを内暈(うちかさ)とよぶ。このほか半径46度の光の輪もときどき現れる。これは外暈(そとかさ)である。内暈と外暈の色の配列は同じで、外側が青、内側が赤である。光の輪のほか、次に述べるような光の弧、玉、柱などが氷晶の屈折や反射ででき、これらをまとめて暈(かさ)現象とよぶ。太陽を通る水平線上で、内暈の上にのるか、またはすこし外側に白色の光の玉が現れることがある。これは幻日(げんじつ)である。幻日を通って斜めに内暈に達する弧が現れることがあるが、これをローウィッツ弧という。内暈や外暈の最高点（または最低点）に接して現れる光の弧を上端接弧（または下端接弧）という。天頂付近に現れる光の弧を天頂弧という。天頂弧の色の配列は外が赤、内がすみれ色で、太陽の高度が32度以下と低いときのみに現れ、美しい虹(にじ)のように見えることがある。太陽を通って鉛直に光の柱が立つことがある。これは太陽柱といい、日没か日の出時に現れやすい。なお、極寒の地では、氷霧(こおりぎり)のときに暈現象が現れることがある。

　暈現象が現れると天気が悪くなるという天気俚諺(りげん)がある。暈現象は巻層雲に出ること、巻層雲は低気圧の前面にできやすいことを考え合わせると、この俚諺は当たることもある。ただし低気圧がどこを通るかによって外れることもある。

［大田正次］

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