Here's why:

* High Altitude: Cirrostratus clouds are high-altitude clouds made of ice crystals.

* Light Refraction: When sunlight or moonlight passes through these ice crystals, it gets refracted (bent).

* Halo Formation: This refraction creates a ring of light, or a halo, around the sun or moon.

Additional Notes:

* Other Phenomena: Cirrostratus clouds can also cause other optical phenomena like sun dogs (parhelia) and mock suns.

* Weather Indicator: Cirrostratus clouds are often a sign that a warm front is approaching, which may bring rain or snow within 12-24 hours.