§   Cloud base between 2000 and 6000 m

§   Composed of small water droplets, supercooled water droplets or ice crystals

§   Consists of extensive greyish or faint bluish stratiform clouds, covering all or part of the sky

§   No significant lumps at the cloud base

§   The outline of the sun or moon cannot be seen when thicker altostratus is present

§   When it thickens, it can produce continuous rain or develop into nimbostratus

§   Usually appears when a cold front or a trough of low pressure approaches