A shining iridescent cloud mesmerized locals as it glowed in the sky over a highway in Thailand.

Footage shows the phenomenon peeking out from behind darker cumulus clouds in Samut Prakan province on June 2.

Cloud iridescence occurs when water droplets or ice crystals diffract sunlight, causing the beam to scatter. The clouds must near the sun and be thin and have similarly sized droplets for the phenomenon to occur.

Although they look similar, cloud iridescence is different from 'fire rainbows' or circumhorizontal arcs.

Circumhorizontal arcs are formed not through diffraction, but through refraction or bending of light. They appear like bands parallel to the horizon, with a colour pattern usually much neater than those of iridescent clouds. Like a regular rainbow, they are red at the top and more indigo or purple at the bottom.

One key difference is that the sun must be high up in the sky - more than 58 degrees above the horizon - for circumhorizontal arcs to occur.