Clouds of the stratus family occur as sheets or layers. They have a variety of forms depending on their altitude and thickness. Higher and thinner forms let more light through and appear white and pale grey. Lower and thicker clouds produce dull weather with reduced light reaching the surface. The extent to which light passes through is reflected in supplementary names:
The main types of stratiform cloud include:
When seen as individual clouds, they clearly display the flat bottom and top surfaces of stratus clouds. While regarded as a low cloud, Stratus layers can reach middling altitudes of perhaps 2km.
These grey examples stand out well against a mostly clear evening sky.
Individual Stratus clouds, Harriseahead, Staffordshire
24th April, 2018
© Lionel Burch
Very often stratus clouds cover a greater area of the sky. In this case they are grey and obscure the sun, hence the suffix opacus, meaning opaque.
While the bases of these clouds is distinctly flat, their tops are not as flat as those of the previous image, which invites the question - when does stratus become stratocumulus? - to which there is no easy answer. It is simply a matter of individual judgement. Perhaps some of the clouds with stronger highlights above Roker Pier might just fall in the Stratocumulus category, as their upper surfaces clearly show some ups and downs.
Stratus opacus, Sunderland
23rd February, 2006
© Paul Swinhoe
On occasions the sky is just simply greyed over with an extensive Stratus opacus cloud sheet. Where it shows virtually no variation in tone and no obvious features, the term nebulosus is used. The situation here is approaching that state, although there is just a hint of some gentle undulation.
Stratus opacus, Seaburn Beach, Sunderland
13th October, 2011
© Paul Swinhoe
On the same day, but looking south, there is a little more variation in the tone of the cloud. At the landward end of the pier, there is an area of brighter sky, allowing light from the sun to be more clearly seen, although the disc of the sun would still be hidden even if it was directly behind the brighter patch. As the cloud is neither completely opaque it is termed perlucidus.
The clouds nearer the lighthouse are still very definitely opacus.
Stratus opacus and perlucidus, Roker Beach, Sunderland
13th October, 2011
© Paul Swinhoe
When stratus cloud sheets begin to break up, or when they are forming, scuds of broken ragged cloud occur, such as in this example. After a cloudy morning, the sun eventually managed to heat up and evaporate parts of the cloud sheet producing these fractus clouds.
Stratus fractus and Cumulus fractus can look very similar, but while the latter show swirling air movements, Stratus fractus does not. It simply moves downwind without any evidence of air currents, but as it does so, its edges may begin to evaporate or gradually grow, if the cloud sheet is reforming.
Stratus fractus clouds, Ross Links, Northumberland
31st July, 2002
© Paul Swinhoe