Polar vortex brings rare nacreous clouds to Britain
© North NewsA rare nacreous cloud spotted over Langwathby in Cumbria
Vivid iridescent clouds have been spotted over Cumbria sparked by bitter weather coming from the North Pole.

The multi-coloured clouds are sometimes known as 'mother-of-pearl' or nacreous clouds because of their shimmering pastel hues which resemble the inner surface of sea shells.

Usually the clouds only form over the poles during winter because the air in the upper stratosphere needs to be at least -78C.

Polar vortex brings rare nacreous clouds to Britain
© North News Another view of the cloud seen in Cumbria
Polar vortex brings rare nacreous clouds to Britain
© North NewsThe Nacreous cloud sighted in Cumbria

But the 100mph wind which circles the North Pole, known as the
polar vortex, is currently hovering above Britain, bringing icy conditions and allowing for the strange phenomenon to occur. A rare cloud was snapped over Langwathby in Cumbria on Thursday evening.

Even with the freezing conditions, the colourful clouds can only be seen at twilight or just before dawn when the sun is just below the horizon.

Jet Stream meandering
© Wiki: Fred the OysterMeanders of the northern hemisphere's jet stream developing (a, b) and finally detaching a "drop" of cold air (c); orange: warmer masses of air; pink: jet stream

Because the clouds are so high - 49,000 - 82,000 feet - the water droplets from which they are made are much smaller than those forming more common clouds.

The smaller droplets scatter light in a different way which is what creates the distinctive luminescent appearance.

Gavin Pretor-Pinney, of The Cloud Appreciation Society said: "They only form during the winter time, when temperatures in the lower part of the stratosphere can become cold enough for ice crystals to form in what is a pretty dry part of our atmosphere.

"Normally, very little water vapour manages to rise beyond the troposphere, which is the lower region of our atmosphere where weather, as we normally think of it, happens.

"At the top of the troposphere is a boundary layer called the 'tropopause', where the temperature profile acts as a lid on any rising air currents, and so capping the development of any clouds.

"But in certain stable atmospheric conditions, waves of air produced by winds flowing over mountains can propagate up through the atmosphere and force moisture to rise up through the tropopause into the stratosphere."

Polar vortex brings rare nacreous clouds to Britain
© BBC The clouds have previously been seen in Edinburgh
The clouds are offically known as 'Polar stratospheric clouds' or PSCs and are often seen in more northerly climes such as Scandinavia and northern Canada.

"They are composed of extremely small ice crystals, which are very regular in size," added Mr Pretor-Pinney.

"This means that they diffract the sunlight significantly as it passes through the cloud, which separates out the light's constituent wavelengths. The result is beautiful mother-of-pearl colours, which give the cloud its other name: nacreous clouds. "

The Met Office advises looking out for the clouds at twilight, although they can also sometimes shimmer in the moonlight.

Polar vortex brings rare nacreous clouds to Britain
© North News The tiny droplets change the way light is refracted, creating a pearlised rainbow effect
"These clouds form when the sun is just below the horizon and illuminates the cloud from below giving the vivid colours," said Nicola Maxey of The Met Office.

"They can be visible for up to a couple of hours after sunset and even through the night as they can be lit by moonlight.

"Nacreous clouds are most likely to occur during the polar winter but are quite rare over the UK.

"They are usually only visible here when the cold upper air which circulates around polar regions is displaced and hovers temporarily over the UK creating very cold conditions.

"Twilight is the best time to see them, when the sun dips just below the horizon."

However behind their iridescent, rainbow-colours the clouds are bad news for the atmosphere and are a major contributing factor in the formation of ozone holes in the Arctic and Antarctic.

The nacreous clouds provide an ideal surface for chemical reactions that involve chemicals from banned CFCs. As these compounds interact with the water droplets in stratospheric clouds, chlorine gas is released.

When sunlight hits chlorine gas the energy breaks the bond between its two chlorine atoms and elemental chlorine is released sparking a chain reaction which destroys thousands of ozone molecules.

Although CFS have now been banned globally, they can last in the atmosphere for 100 years so can still be causing damage.