Tuesday, 21 February 2017

'Paddle Pop' rainbow spotted in Singapore skies

Update 26 Feb 2017: Sorry Singapore, what you saw wasn't a ‘fire rainbow’
An iridescent cloud seen from Carmichael Road on Mon, Feb 20, 2017. Foto courtesy of Zhou Guang Ping

When a gorgeous multi-coloured glow lit up parts of the skies over the Republic on Monday (Feb 20), many media reports and netizens labelled the rare weather phenomenon a “fire rainbow”.

But the Meteorological Service Singapore (MSS) & a geography expert have told TODAY that the unusual sight was not a fire rainbow, or circumhorizontal arc as it is officially known.

Instead, what many Singaporeans saw was an iridescent cloud – a thin layer cloud formed when a humid layer of air above a low-level cumulus cloud is pushed upwards by the cotton-ball shaped cloud growing & rising.

Differences between an iridescent cloud and a fire rainbow (or circumhorizontal arc)
Iridescent cloud

fire rainbow (or circumhorizontal arc)

Circumhorizontal arcs, especially when only fragments can be seen, are sometimes confused with cloud iridescence. This phenomenon also causes clouds to appear multi-coloured, but it originates from diffraction (typically by liquid water droplets or ice crystals) rather than refraction.

The two phenomena can be distinguished by several features:
  • A circumhorizontal arc always has a fixed location in the sky in relation to the Sun or Moon (namely below it at an angle of 46°), while iridescence can occur in different positions (often directly around the Sun or Moon).
  • The colour bands in a circumhorizon arc always run horizontally with the red on top, while in iridescence they are much more random in sequence and shape, which roughly follows the contours of the cloud that causes it.
  • The colours of a circumhorizon arc are pure and spectral (more so than in a rainbow), while the colours in cloud iridescence have a more washed-out, "mother of pearl" appearance.

Incredible 'fire rainbow' cloud phenomenon creates stunning sky display
A spectacular 'fire rainbow' cloud phenomenon has been captured on camera lighting up the sky in Singapore

The amazing sight was visible over the sovereign city-state for around 15 minutes on February 20, one witness told the BBC. “It started as a small orange circle then grew bigger and bigger [until] all the colours came out … It lasted for about 15 minutes and it slowly went off,” explained a childcare worker Fazidah Mokhtar.

The phenomenon is known as cloud iridescence, weather.com’s senior meteorologist Nick Wiltgen previously explained:

  • "In clouds, iridescence is a by-product of sunlight being diffracted by water droplets or ice crystals, causing the various wavelengths of light, which we see as colours, to emerge at different angles," he said.
  • "As they reach the observer's eye, the observer perceives a pattern of various colours as those different wavelengths reach his or her eye from distinct directions, rather than being jumbled together and appearing whitish."

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Stunning 'fire rainbow' lights up the sky over Singapore
Singapore was treated to a rare weather phenomenon as an apparent "fire rainbow" lit up the sky

Weather watchers on the island state in south-east Asia were treated to the multi-coloured glow yesterday. The stunning scene may also have been cloud iridescence. Both phenomena can be caused by the refraction of light through ice crystals, or in the case of iridescence, by water droplets.

For iridescence to occur clouds must be thin so the sun's rays encounter very little water. The technical name for a fire rainbow is a "circumhorizontal arc".

The light show persisted for about 15 minutes and could reportedly be seen across the island.

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'Paddle Pop' rainbow spotted in Singapore skies
The ‘Paddle Pop’ rainbow spotted in Singapore on Monday (20 February) received a lot of social media attention. (PHOTO: howenrui / Instagram)

A “delicious”-looking weather occurrence was spotted in the Singapore sky on Monday (20 February), drawing much attention from social media users.

Dubbed a “Paddle Pop” rainbow by netizens – in reference to the popular rainbow-coloured frozen dairy snack – some speculated that it could have been a fire rainbow, a rare weather phenomenon caused by clouds of water droplets of uniform size. The term “fire” in its name is said to come from how fire rainbows appear in arcs resembling flames in the sky.

Whatever it may have been, it was surely a colourful way to start the week.

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Singapore 'fire rainbow' cloud phenomenon lights up sky
A rare cloud phenomenon over Singapore has delighted people in the city-state

The multi-coloured glow appeared in the sky on Monday in the late afternoon, lasting for about 15 minutes, and was seen across the island.

Media reports said it was likely a fire rainbow, which occurs when sunlight refracts through ice-crystal clouds.

Others have also said it could have been cloud iridescence, which happens when water droplets or crystals scatter light.

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Rare 'fire rainbow' spotted over Singapore
An unusual rainbow caught the attention of residents in several parts of S'pore on Mon afternoon (Feb 20)

Many of them took to social media to share photos of the dazzling sight, with some suggesting that it could be a fire rainbow.

A fire rainbow is an optical phenomenon also known as circum-horizontal arc, according to a Facebook post by National Environment Agency (NEA) last April.

"They’re actually ice halos formed by the refraction of the sunlight (or occasionally moonlight) in plate-shaped ice crystals suspended in the atmosphere,” explained NEA.

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Cloud iridescence

Cloud iridescence is the occurrence of colors in a cloud similar to those seen in oil films on puddles, located in the general vicinity of the sun or moon. It is a fairly common phenomenon, most often observed in altocumulus, cirrocumulus, lenticular clouds and cirrus clouds. The colors are usually pastel, but can be very vivid. When occurring near the sun, the effect can be difficult to spot as it is drowned in the sun's glare. This may be overcome by blocking the sun with one's hand or hiding it behind a tree or building. Other aids are dark glasses, or observing the sky reflected in a convex mirror or in a pool of water.

The effect is similar to irisation. Iridescent clouds are a diffraction phenomenon caused by small water droplets or small ice crystals individually scattering light. Larger ice crystals do not produce iridescence, but can cause halos, a different phenomenon.

If parts of clouds contain small water droplets or ice crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, while newly forming clouds produce the brightest and most colorful iridescence. When the particles in a thin cloud are very similar in size over a large extent, the iridescence takes on the structured form of a corona, a circular bright disk around the sun or moon, surrounded by one or more colored rings.

Circumhorizontal arc (Fire Rainbow)

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sun or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band (red being the topmost colour) running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22-degree halo. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.

Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc. The misleading term "fire rainbow" is sometimes used to describe this phenomenon, although it is neither a rainbow, nor related in any way to fire. The term, apparently coined in 2006, may originate in the occasional appearance of the arc as "flames" in the sky, when it occurs in fragmentary cirrus clouds.

The halo is formed by sunlight entering horizontally-oriented, flat, hexagonal ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo). In principle, Parry oriented column crystals may also produce the arc, although this is rare. The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colours. The arc has a considerable angular extent and thus, rarely is complete. When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colours

Christians see ‘fire rainbow’ as divine approval of Netanyahu’s visit to Singapore

Some Christians in Singapore have linked a ‘fire rainbow’ which was visible in many parts of Singapore yesterday with divine approval of Singapore’s close ties with Israel.

The multi-coloured fire rainbow which appeared for over 15 minutes across the island is a rare phenomenon which occurs when sunlight refracts through ice-crystal clouds.

The appearance of the fire rainbow coincided with the first visit of a head of government from Israel in over 2 decades.

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