This is the spectacular moment a hat-shaped cloud iridescence appeared in the sky in southern Thailand on June 20.

A rainbow cloud, also known as a Circumhorizontal arc, forms because of something called cloud iridescence. This is also known as "irisation."

This is the spectacular moment hat-shaped cloud iridescence appeared in the sky in southern Thailand.

Onlooker Orawan Thongchinda recorded the weather phenomenon while heading back home in Narathiwat province, on June 20.

She said: ‘I remember feeling happy to have come across something so beautiful. I’ve never seen anything like it before.’

A rainbow cloud, also known as a Circumhorizontal arc, forms because of something called cloud iridescence. This is also known as ‘irisation’. Both terms come from the word ‘Iris’, which is the Greek personification of the rainbow.

This only happens when clouds are thin. They are usually altocumulus, cirrocumulus, lenticular and cirrus clouds. Small water droplets or crystals are suspended in the clouds. For the iridescence to happen, the water droplets or crystals should be around the same size.

The angle is important, too. Water droplets should be about 10 degrees from the Sun. So rainbow clouds form when the sun is relatively low, late in the afternoon. The iridescent clouds form because of diffraction.

The sun shines through the cloud and the light is scattered and reflected through the clear water droplets like a prism. Sometimes the effect is spectacular. All of the colours of the rainbow can be seen hovering over the edge of the cloud. Sometimes the rainbow clouds can resemble giant spaceships floating in the sky.