While stratus forms like fog, when stable air near the ground is moist enough and cool enough, altostratus normally forms when moist, stable air is lifted up to its condensation level. The lifting usually happens when the air mass rises smoothly up the slope of a warm front.If altostratus gets thick enough, rain can fall out of it. At times the rain disappears before it reaches the ground, and is then called virga (from the Latin for twig or branch.) If the rain strengthens and persists, the cloud is renamed nimbostratus. Since it’s associated with frontal weather, it can be accompanied by other cloud types, including embedded thunder clouds.
While similar to stratus, altostratus is more complex.