Altostratus occurs as sheets or layers, often in assocation with fronts, particularly warm fronts. As with many other groups they include opacus, perlucidus and translucidus varieties, depending on the amount of light they allow through.
The main types of altostratus include:
These are the thinnest and higest of the altostratus clouds, thin enough to allow the sun to shine through, albeit very weakly. This can been by looking at the column in the foreground, which has the faintest of shadows cast behind it to the left.
There is no variation in the tone of this particular cloud sheet, assigning it to the species nebulosus.
Altostratus nebulosus translucidus, Melrose Abbey
22nd July, 2002
© Paul Swinhoe
Here the cloud is still in its featureless nebulosus form, but it is thicker, and there is no hint of any sunlight producing showers. It was a drab afternoon as a warm front advanced into the area, eventually producing rain later in the day.
Altostratus nebulosus opacus, Maer Hills, Staffs
18th November, 2001
© Lionel Burch
Relatively low altostratus is also often a feature of the trailing edge of cold fronts, as in this case where the transition from opacus into perlucidus, in which the areas where the sun is shining through are clearly seen.
Altostratus perlucidus, Harriseahead, Staffordshire
31st December, 2013
© Paul Swinhoe
Viewed from a different angle, this trailing edge of an occluded front shows the change from darker altostratus opacus along the horizon, and perlucidus clouds above, through which the sun is shining, whilst its disc cannot be seen.
Altostratus perlucidus, Wedgwood Memorial, Staffordshire
23rd December, 2009
© Paul Swinhoe