Published on 2 Feb 2016

Wondering what those mother of pearl clouds are?

During the past few days, lots of you have been asking me on Twitter about some rather unusual clouds that you’ve spotted in the sky during the mornings and evenings.

They are called nacreous clouds or polar stratospheric clouds and occur in the winter polar stratosphere (upper part of the atmosphere) at an altitude of 15-25km.

SALTBURN-BY-THE-SEA, ENGLAND - FEBRUARY 02: A man stands on top of a hill as ice particle iridescence is seen in high-level Nacreous clouds on February 02, 2016 in Saltburn-by-the-Sea, England. The rare effect is caused by the diffraction of sunlight in the ice crystals that form the Nacreous clouds at high altitudes. (Photo by Ian Forsyth/Getty Images)

The high clouds, which glow brightly with a variety of colours, have a similar appearance to mother of pearl – due to their iridescent nature.

They are most likely to be seen during civil twilight when the sun is 1-6 degrees below the horizon, because sunlight illuminates them from below, giving them their colourful glow.

The reason that they are so unusual is that they form in the stratosphere which is very dry and normally lacking enough moisture to form any clouds.

However, during the polar winter, there is occasionally enough moisture present to form clouds in the stratosphere, giving pretty, colourful displays.

The clouds form at very low temperatures – often below minus 78C – which is why they form during the winter season.

Given the prevalence of sightings during the past few days, they could well be spotted again. If you manage to capture any pictures, you can send them to me on Twitter – @liamdutton

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