Cloud Classification


General description

Generally grey cloud layer with a fairly uniform base.  It usually occurs below about 2000ft AGL. When the sun is visible through the cloud, its outline is clearly discernible. Stratus does not produce halo phenomena except possibly at very low temperatures.

Common subdivisions

The species of Stratus described above is Stratus nebulosus.

Sometimes stratus appears in the form of irregular ragged shreds. This species is known as Stratus fractus. It usually forms beneath the base of higher precipitating cloud. In such an instance it is known as Stratus fractus of bad weather. Despite the name, Stratus fractus of bad weather does not itself produce precipitation.

Distinguishing St from other genera

Stratus is distinguished from Altostratus by the fact that when the sun is visible it does not blur its outline.

Thick Stratus differs from Nimbostratus in that its base is more clearly defined and uniform, and it can produce drizzle, as opposed to rain from Nimbostratus.

Stratus is distinguished from Stratocumulus in that it shows no evidence of elements, either merged or separated.

Stratus fractus is less white and less dense, with smaller vertical development, than Cumulus fractus.

Associated precipitation

The precipitation associated with Stratus nebulous is drizzle when sufficiently thick. It can also produce snow and snow grains.

Stratus nebulosus
Stratus nebulosus (Photo P. Toomey)
Stratus fractus
Stratus fractus beneath Altostratus (Photo: P. Toomey)
Stratus nebulosus
Stratus nebulosus (Photo: P. Leigh)