Is it an aurora? Is it a UFO? No, it's a nacreous cloud! I expect these rare shimmering "rainbow" clouds have mesmerised thousands of you in the last couple of days at dawn and dusk, but what exactly are they?

After a sleepless night (blame Storm Henry), I opened the blinds and witnessed my first nacreous clouds. Great excitement!

I ran out of the house still in my dressing gown and into the garden to frantically take a picture of it before it disappeared. I'm sure my neighbours must think I'm a bit mad, as this is not the first time I've stood in the garden in my pjs watching weather or astronomical events.

It was a beautiful sight - vivid, shimmering and majestic. It seemed to change colour very quickly as the sun came up, and I now realise why some people mistake them for the northern lights.

Here's a photo I managed to take from my back garden in Glasgow.

While my photo isn't so great, here's some spectacular photos that were emailed to me this morning.

Lots of people tweeted me during sunrise with their photos. Most of the pictures came in from the Dundee, Edinburgh and Fife areas where the skies were clear to start the day.

You may notice from the pictures that the same three clouds make an appearance in a few shots from Dundee and Edinburgh. The reason for this is that they can be seen from two places, about 40 miles apart, as they sit so high in the sky.

Nacreous clouds are in the stratosphere about 15 miles up. This is twice as high as commercial planes fly. They are a little more common the further north you go, over the likes of Scandinavia and northern Canada. To see them this far south is quite rare.

Nacreous form at temperatures below -78C and are made up of tiny water droplets, which scatter the light. The light is scattered in such a way that the cloud takes on a pearly appearance when the sunlight hits them at a low angle during sunrise and sunset. This is why they are sometimes referred to as mother of pearl clouds.

Because they only form in such frigid conditions, the clouds usually occur between December and February in the northern hemisphere, when the stratosphere is at its coldest.

Storms also help in its formation, as they create a very turbulent flow in the lower atmosphere which can inject a little moisture into the lower stratosphere, which is generally far too dry for cloud formation.

Although the storms have passed, temperatures of the lower stratosphere over Scotland should remain low enough for a few days, before rising a little later in the week. This means nacreous clouds may still be present, and thanks to clearer skies for much of the country on Wednesday morning, a few more people may be able to spot these little morning wonders.

While the clouds put on a pretty light show, they have a more sinister side. They create a chemical reaction which destroys the ozone layer.

For example, during the dark Antarctic winter it is cold enough for nacreous (ice clouds) to form in the lower stratosphere. The presence of these clouds is key for the chemical changes that produce destructive chlorine.

When sunlight returns to the Antarctic in the spring, the combination of sunlight and active chlorine leads to rapid chemical ozone loss, causing the hole.