Weird Science

Circumzenithal arc

Polish ver­sion is here

Nature rarely reve­als its most cap­ti­va­ting facets to those who keep their gaze fixed on the gro­und. We are accu­sto­med to sear­ching the sky for rain­bows arcing across distant clo­uds, yet we sel­dom con­si­der loo­king almost stra­i­ght upward toward the zenith on a rain­less day. It is pre­ci­sely there, high in the upper lay­ers of the tro­po­sphere, that one of the most refi­ned spec­tac­les of atmo­sphe­ric optics quie­tly takes shape: the cir­cum­ze­ni­thal arc (CZA). Often descri­bed as “the smile in the sky,” its curva­ture is inver­ted rela­tive to a rain­bow, imme­dia­tely set­ting it apart as a distinc­tive mem­ber of the halo family.

Ilustracja

Unlike a rain­bow, which owes its exi­stence to the refrac­tion and inter­nal reflec­tion of light within water dro­plets, the cir­cum­ze­ni­thal arc forms exc­lu­si­vely in the pre­sence of ice cry­stals. These cry­stals must exhi­bit a highly spe­ci­fic geo­me­try, appe­a­ring as thin hexa­go­nal pla­tes. Equ­ally cri­ti­cal is their sta­ble, hori­zon­tal orien­ta­tion within calm, tur­bu­lence-free air, con­di­tions most com­monly enco­un­te­red in cir­rus clo­uds. Within this pre­cise opti­cal con­fi­gu­ra­tion, sun­li­ght enters a cry­stal thro­ugh its upper hori­zon­tal face, under­goes refrac­tion inside, and then exits thro­ugh one of the ver­ti­cal side faces. The angle between these sur­fa­ces mea­su­res 90°, a geo­me­try that pro­du­ces excep­tio­nally strong disper­sion by effi­cien­tly sepa­ra­ting light into its con­sti­tu­ent wave­leng­ths. The resul­ting colors display a satu­ra­tion and purity rarely mat­ched by other atmo­sphe­ric phe­no­mena, rain­bows inc­lu­ded. Vio­let appe­ars clo­sest to the zenith, while red is direc­ted toward the lower-posi­tio­ned Sun.

The appe­a­rance of a cir­cum­ze­ni­thal arc is tigh­tly con­stra­i­ned by the geo­me­try of the Sun’s posi­tion. The phe­no­me­non can occur only when the Sun lies below 32.5° above the hori­zon. Bey­ond this limit, total inter­nal reflec­tion traps the light within the cry­stal, pre­ven­ting the arc from for­ming. The most stri­king displays occur when the Sun rea­ches an alti­tude of about 22°, a con­fi­gu­ra­tion that opti­mi­zes both bri­ght­ness and color sepa­ra­tion. As the Sun climbs higher, the radius of the arc ste­a­dily decre­a­ses. When the Sun hovers just above the hori­zon, the arc spans rou­ghly 32°, but as the cri­ti­cal thre­shold of 32.5° is appro­a­ched, it the­o­re­ti­cally con­tracts to a sin­gle point at the zenith before vani­shing from view.

Obse­rva­tions

Janu­ary 9, 2026, aro­und 12:30 PM – Dąbrowa Gór­ni­cza (Poland)
urban envi­ron­ment

Retur­ning in good spi­rits from a pro­duc­tive ses­sion with my stu­dents, I had lit­tle rea­son to expect that I was about to wit­ness a stri­king atmo­sphe­ric phe­no­me­non. When I lifted my gaze, howe­ver, a splen­di­dly defi­ned cir­cum­ho­ri­zon­tal arc came into view over­head, its colors vivid and shar­ply deli­ne­a­ted aga­inst the sky (Photo 1).

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Photo 1

The phe­no­me­non was visi­ble in its most vivid, color­ful form for 4-5 minu­tes. It then faded, per­si­sting as a pale, whi­tish arc of light for the next fifteen minu­tes or so.



Janu­ary 18, 2026, aro­und 13:20 PM – Jaworzno (Poland)
urban envi­ron­ment

Thanks to favo­ra­ble con­di­tions, I was able to obse­rve ano­ther beau­ti­fully colo­red cir­cum­ze­ni­thal arc (Photo 2).

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Photo 2

Discus­sing atmo­sphe­ric optics is a recur­ring remin­der that the sky above us is a vast, natu­ral labo­ra­tory for phy­sics. Whe­ne­ver deli­cate, fea­ther-like clo­uds drift across the sky, it is worth loo­king a lit­tle higher than usual and allo­wing one­self a moment of atten­tive obse­rva­tion. Under the right con­di­tions, such a glance may be rewar­ded with one of nature’s most sub­tle and cap­ti­va­ting displays.

Fur­ther rea­dings:

Marek Ples

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