Altostratus
§ 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