The altostratus is one of the middle cloud with height between 2000 and 6000 meters. This cloud is semi-transparent or light-shielded. The former is thinner and can be penetrated by sunlight and can also see the sun or the moon outline. The latter are thicker and light cannot penetrate the clouds to see the sun or the moon. When the clouds are thick, the sky is dark and often accompanied by continuous precipitation. Altostratus clouds are composed of small water droplets and ice crystals. If the cloud height above the melting layer (temperature 0℃), it is mainly ice crystals.