Height of base:

Between 2000 and 6000 m (6500 - 18000 ft) (mid-level clouds)

Description:

Featureless grayish-blue cloud layer, occasionally fibrous

Extended over large areas

Often reveal position of sun in coarse image (like trough ground glass)

Can cause white or colored corona

“the boring cloud”

Occurrence:

World-wide; common in midlatitudes

Made of:

Supercooled water droplets and ice crystals

Precipitation:

None; rarely light rain or snow

Formation:

Follows Ac in approach of warm front

Forming from thickening and lowering cirrostratus

Varieties:

Opacus, translucidus, undulatus, duplicatus, radiatus

How to distinguish from …

Cirrostratus (Cs)

Cs produces haloes, As does not; but both can produce coronae

Sun through Cs can produce shadow, As does not

Stratus (St)

St is lower, often some features can be distinguished on its base; As is completely featureless

Nimbostratus (Nb)

Darker; produces heavier precipitation

Altostratus translucidus in upper

Part of image

(Morris, October 2007)

Altostratus opacus

(Morris, October 2007)

Altostratus and altocumulus,

Morris, April 24, 2008

Altostratus translucidus layer and cumulus fractus

Morris, March 28, 2008