The highest clouds in the sky are called noctilucent clouds (NLCs). They're made of ice crystals that form in the mesosphere, a layer of the atmosphere much higher than where regular clouds form.

Here's why they are special:

* Extreme altitude: NLCs appear between 76 and 85 kilometers (47 and 53 miles) above Earth, far higher than any other cloud type. This puts them in the mesosphere, a very thin and cold region.

* Composition: These clouds are made up of tiny ice crystals that form around dust particles. These dust particles come from various sources, including meteoroids and volcanic eruptions.

* Appearance: NLCs are often described as shimmering, blue-white, or silver clouds that appear in the twilight sky.

While they are fascinating, they are also a sign of change in the Earth's atmosphere, potentially linked to climate change and its impact on the upper atmosphere.