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Out of the blue: Sky events around Singapore dazzle onlookers, but are they related?

Double rainbows, supermoons, iridescence and other phenomena in the sky are unrelated events

Something seems to be up with the sky these days - what with a supermoon that is more super than others, unusual rainbows and pink and purple sunsets bathing the city.

In November last year, the world saw the biggest supermoon in almost 70 years.

Twice in the last six months - on Sept 30 last year and Jan 30 this year - double rainbows spanned the skies of Singapore after it rained. The second one appeared on the third day of Chinese New Year, to the delight of some people. In Chinese culture, a double rainbow is considered auspicious.

Then, on the afternoon of Feb 20, sightings of what was initially thought to be a fire rainbow - resembling Paddle Pop ice cream - took social media by storm. The Meteorological Service Singapore (MSS) later clarified that it was not a fire rainbow.

In the last few months, Singaporeans have also been treated to some pink and purple-hued sunsets. According to citizen journalism website Stomp, one of those sunsets took place on the evening of Feb 4.

So, what is happening up there?

Experts whom The Sunday Times spoke to say the various occurrences are not related and that they happen for different reasons.

Associate Professor Koh Tieh Yong, a weather scientist from UniSIM, notes that the supermoon is not a meteorological phenomenon, but one of celestial mechanics: It is a full moon that is at its nearest distance from Earth, so it appears bigger and brighter than usual.

"A supermoon's occurrence follows a regular pattern according to the periodicities of the moon's orbit around Earth and Earth's orbit around the sun," he says. This results in four to six supermoons in a year on average, he adds.

As for the double rainbows and iridescence, an MSS spokesman says they are common atmospheric optical phenomena formed when sunlight, passing through the atmosphere, is scattered, diffracted, reflected or refracted, producing a colourful appearance in the sky.

A rainbow is formed when sunlight is refracted and reflected against raindrops in the sky. And a double rainbow appears when light is reflected twice inside the raindrop instead of once.

As for a fire rainbow, it is formed when sunlight is refracted by ice crystals in high-altitude cirrus clouds. Unlike rainbows, iridescence is caused by diffraction (when light bends). In iridescent clouds, sunlight bends around cloud water droplets or ice particles rather than passing through them.

According to the MSS spokesman, double rainbows and iridescence are naturally occurring phenomena and may not always happen.

Assistant Professor Winston Chow from the National University of Singapore's geography department adds: "It's more likely that the specific weather conditions were just right for both to occur."

For instance, he notes that three conditions are needed for a double rainbow to occur: a dark, cloudy and rainy sky in the distance; the sun at a low angle (early morning or late afternoon); and the person seeing the rainbow has to be in a place where the sky behind and above him is clear so that sunlight can pass through unimpeded to be reflected or refracted back to him.

"The key to seeing the double rainbow is the sharp contrast between sunny and rainy locations ," he says.

While the science behind these phenomena may take the layman a while to grasp, it is good news for shutterbugs such as professional photographer Jacqueline Choo, 36. She did not catch the double rainbows and the iridescent clouds, but loves the "beautiful pink and purple-hued sunsets".

Prof Koh says sunsets are most colourful when the sky is clear.

Ms Choo says: "That and the recent photos of cloud iridescence have inspired me to look skywards for photo opportunities."


Double rainbow


A double rainbow seen over Singapore on Jan 30. PHOTO: COURTESY OF TERENCE YEO

When: Sept 30 last year and Jan 30 this year
Where: Most sightings were reported in central Singapore and the Central Business District.
How: Rainbows are formed when sunlight is refracted and reflected inside raindrops in the sky.

A double rainbow is the result of light undergoing reflection twice inside the raindrop, instead of once, giving rise to a primary rainbow from one internal reflection and a secondary rainbow from two internal reflections.

This causes the secondary rainbow - which is above the primary one - to have fainter colours that are in reverse order.


Supermoon


A photo of the moon taken in Bedok Reservoir Road on the morning of Nov 15 last year, the day after the supermoon occurred. PHOTO: JEFFREY LIM

When: Nov 14 last year
Where: All around the world
How:The term supermoon was coined by American astrologer Richard Nolle in 1979. 

He defined a supermoon as a new or a full moon that occurs when the moon is within 360,000km from Earth.

As the moon is closer to Earth than usual, it can visually appear bigger than usual in clear weather, says Mr Albert Ho, president of The Astronomical Society of Singapore. 

The one in November was special because it was said to be the closest full moon - at a distance of 357,000km - to Earth since 1948. It appeared up to 14 per cent bigger and 30 per cent brighter than a full moon when it is farthest from Earth, says Mr Ho.

There will not be another like it until 2034.

Unfortunately, due to overcast weather in Singapore, the moon could not be seen at all on Nov 14.

Some people said they were able to see it clearly when the weather cleared the next day.

Mr Ho points out that there is little or no significant difference in the appearance of the moon from that of the supermoon within a day or two of the phenomenon.


Iridescent clouds


An iridescent cloud seen on Feb 20. PHOTO: AARON TOH

When: Feb 20
Where: Several parts of Singapore in the afternoon
How: In iridescent clouds, sunlight bends or diffracts around cloud water droplets or ice particles rather than passing through them.

The light then recombines to give various pastel colours, depending on the size of the water droplet in the cloud. Small cloud droplets of the same size produce the best visual effect, says the Meteorological Service Singapore.

However, according to Associate Professor Koh Tieh Yong from UniSIM, most clouds consist of droplets in a wide range of sizes. Hence iridescent clouds are rare. In tropical climates, most clouds tend to be thick, making iridescence even rarer.

The iridescent cloud seen on Feb 20, says Prof Koh, is the result of a transient plume of condensation above the main cloud mass - called a convective overshoot - being unusually thin and comprising uniformly small droplets.


Pink and purple-hued sunsets


The sunset seen at East Coast Park on Feb 20. PHOTO: JACQUELINE CHOO

When: Some evenings in the last two months
How:Sunsets are most colourful when the sky is clear, says Associate Professor Koh Tieh Yong from UniSIM.

When there are fewer clouds - for instance, at the end of a sunny day - the colour contrasts are greatest.

The colours come from the scattering of light by nitrogen and oxygen molecules in the air.

The few clouds that are present on the horizon can help reflect the yellow, orange and red hues, and accentuate the colours.

But when too many clouds are present, they block the sun and not much colour can be seen.

A version of this article appeared in the print edition of The Sunday Times on March 05, 2017, with the headline 'Out of the blue'. Print Edition | Subscribe