AN Ystwyth valley astronomer has captured a rare phenomenon that happens at dusk.

Kris Fry captured these stunning images of Nacreous Clouds -also known as rainbow clouds - from the Trawscoed Valley on Thursday afternoon.

Kris said: "These rare Nacreous Clouds were imaged from our garden in the Trawscoed Valley near Aberystwyth on 21 December at around 4pm-4.30pm.

"The first image is 'Welsh Pony Cloud' with a nacreous backdrop!"

Nacreous Clouds are an extremely rare sight in Britain - you can only ever see them at sunrise or sunset in the wintertime when the sun is low in the sky or below the horizon.

They occur at about 70,000 to 100,000 feet above the earth's surface. That's above where our everyday weather occurs in an area called the stratosphere.

The clouds are made of minute ice crystals which act like prisms refracting and reflecting the sunlight from below to produce what is known as cloud iridescence.

Due to their high altitude and the curvature of the Earth’s surface, these clouds are lit up by sunlight from below the horizon and reflect it to the ground, shining brightly well before dawn and after dusk.

They are most likely to be viewed when the Sun is between 1º and 6º below the horizon and in places with higher latitudes, such as Scandinavia and northern Canada. For this reason, they are sometimes known as polar stratospheric clouds. Nacreous clouds only form below -78 °C so are most likely to occur during the polar winter.

Because of the very low temperatures required, nacreous clouds are usually only visible from the UK when the cold air which circulates around polar regions in the stratosphere (known as the stratospheric polar vortex) is displaced and hovers temporarily over the UK.