Therefore, the clouds were unusually beautiful over northern Sweden

They have been called the crown jewel of the clouds. The unusually beautiful pearl clouds that shimmer in the colors of the rainbow.

During the weekend they appeared in several places in northern Sweden. Several viewers have heard about SVT's meteorologists and wondered what they really are.

Pearl clouds are formed high up in the atmosphere at 20-30 kilometers altitude - unlike ordinary clouds formed at about 1-10 kilometers altitude. They consist of tiny ice or water particles as well as sulfuric acid and nitric acid.

When they are visible it is usually at dawn or dusk and always at the poles. The reason is that it needs to be below -75 degrees for the clouds to form.

- They don't show up often. It is partly required that they are no clouds below and that it blows a lot at high altitude. But they are beautiful, says Tora Tomasdottir, SVT's meteorologist.

She continues:

- These are clouds that are almost twice as high as normal clouds. The fact that there is something up there at all makes it interesting for a meteorologist as there is not so much moisture or anything else there.

But even though they are beautiful, the clouds are not only positive. When the clouds are formed, the degradation of the ozone layer increases simultaneously.

The clouds are considered to be a reason why ozone holes can occur at the poles.