The type of high-level cloud that is very thin and causes a halo effect around the sun or moon is called a cirrostratus cloud.

Here's why:

* High-level: Cirrostratus clouds are found at high altitudes (above 6,000 meters or 20,000 feet).

* Thin and wispy: They are composed of ice crystals and are so thin that they often appear almost transparent.

* Halo effect: The ice crystals in cirrostratus clouds refract sunlight, causing a halo or ring to appear around the sun or moon.

Let me know if you have any other questions!