§   Cloud base from near the ground to 2000 m high

§   An extensive thick and dense piece of cloud with horizontal extent from tens to a few thousand kilometers with no obvious boundaries

§   Dark grey in appearance; completely obscures the sun

§   Scattered ragged cloud patches usually appear near and below the cloud base

§   Contains a large amount of moisture and often accompanied by continuous precipitation

altostratus image

§   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

altocumulus image

§   Cloud base between 2000 and 6000 m

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

§   Sometimes appear as flat round cotton balls, sometimes in scaly forms

§   Size and spatial distribution of cloud lumps are more uniform

§   Cloud lumps often appear in groups or tiles

§   Visual angle of each lump usually between 1 and 5 degrees

Go to top