Altostratus clouds are mid-level clouds that form at an altitude of approximately 6,500 to 20,000 feet (2,000 to 6,000 meters). They are generally gray or blue-gray in color and cover the entire sky, but are not as thick and dense as stratus clouds.
Altostratus clouds are typically associated with stable weather conditions, and they often form ahead of a storm system as warm, moist air rises and cools, leading to the formation of these clouds. They can also form as a result of lifting air over mountains or as a result of large-scale air movements in the atmosphere.
Altostratus clouds can sometimes produce light precipitation, such as drizzle or light snow, but this is not always the case. They can also create a diffuse, hazy appearance to the sky, which can cause the sun or moon to appear as if they are shining through a thin veil.