Rainbows reveal their true colours

PHOTOS: NASA, FLICKR USER BLUESKYFANTASIE, FLICKR USER SERA TU, GABY ALTENBERGER, NICOLE CAMBERT

A rainbow in its vastness and vibrance is linked with prosperity in many cultures. People still talk about the pot of gold at the end of a rainbow, a myth long debunked.

Still, to exist, a rainbow needs something almost as shiny - water droplets or ice crystals.

Light is reflected, refracted (bent) and dispersed into its constituent colours, to form distinctive coloured bands in the sky.

Each band represents a range of wavelengths in the visible light spectrum, with light in the red spectrum having the longest and violet the shortest.

The rainbow's appearance depends on prevailing conditions such as the angle of the sun to the horizon, the humidity of the air, and whether there are hills or buildings around.

Typically, a rainbow is semicircular. Generally, the sun must also be less than 42 deg high in the sky for a rainbow to be seen.

According to researchers from France, who concluded an extensive study from photographs in 2015, there are 12 different types of rainbow. Some sport all the colours of the visible light spectrum; others are monochromatic rainbows showing only red, for instance.

Last month, an unusual iridescence in Singapore skies took social media here by storm, with some calling the phenomenon a Paddle Pop rainbow.

Here is a selection of some of the more fascinating rainbows.

CIRCUMHORIZONTAL ARCS

Sometimes called fire rainbows, circumhorizontal arcs are distinguished by the conditions required for their formation. Formed by sunlight refracted off flat ice crystals that are parallel to the ground, they require the sun to be 58 deg above the horizon. Such crystals can be found in thin cirrus clouds at high altitudes.

RED RAINBOWS

These rainbows are dominated by a red colouration rather than several distinct bands. They are formed in the evening, when longer-wavelength red light penetrates through atmospheric dust and air molecules more easily than shorter-wavelength blue or indigo light.

DOUBLE RAINBOWS

This phenomenon occurs when light is reflected a second time off the back of a water droplet. The fainter rainbow, sometimes called the secondary bow, can be found approximately 10 deg away from the primary bow.

CIRCULAR RAINBOWS

Such rainbows are rare, visible only given certain weather conditions, a low sun height (which makes the arc higher) and a high observer position. They are observed mostly by pilots. Sometimes, they can be seen from a cliff, as shown in this shot taken at Victoria Falls in Zambia.

A version of this article appeared in the print edition of The Straits Times on March 03, 2017, with the headline 'Rainbows reveal their true colours'. Print Edition | Subscribe