Altostratus clouds occur higher in the atmosphere, between 10,000 and 20,000 feet (3,048 and 6,096 meters) up.
They frequently form on top of a warm front moving in on top of a cooler mass of air. The cool air mass acts the same as land in the production of stratus clouds.
From a person’s vantage point on Earth, altostratus clouds can cover the whole sky. They are generally only 1,300 to 5,000 feet (396 to 1,524 meters) thick, but can range over a 600-mile (960-kilometer) wide area.
If the water droplets in altostratus are on the large side, they can produce a cloud cover that either obscures sunlight or blocks the Sun from view entirely.