Stratus

Cumulus clouds have a variety of forms and sizes, depending on the amount, strength and depth of convection:

• Cirrostratus
• Altostratus
• Stratus undulatus
• Nimbostratus
• Stratocumulus

Cirrostratus

The highest of the stratus clouds is cirrostratus. It occurs as thin layers at heights of around 5 to 10 km, and is formed from ice crystals. During the day it can give the sky a milky white appearance. The cloud is not thick enough to obscure the sun or the moon, both of which can produce halos. Cirrostratus is often the first indication of an approaching warm front. In the top left of this photograph there is also a clearer and greyer layer of lower level stratus cloud.

Altostratus

Altostratus is slightly thicker and lower than cirrostratus.  It is made from water droplets rather than ice crystals, so it does not produce halo phenomena.

When it is thin, as in the case opposite, the sun can still shine through it and produce shadows, hence its classification as 'translucidus'.

The sky has a fairly uniform white appearance.  The occurrence of altostratus is usually a sign of the approach of a warm front.

With time it tends to thicken, become grey and cut out the sun, and may produce light rain, before it becomes thick enough and low enough to be classified as nimbostratus.

The grey featureless sky in the picture opposite, shows an example this thicker altostratus.  The term 'opacus' means that the sun is completely obscured.  The term 'nebulus' means 'featureless'.  Unfortunately, we experience a lot of dismal days like this in the winter half of the year in Britain (and a fair few in the summer!)

 

Altostratus & Cirrostratus

Altostratus and cirrostratus often occur together near the leading edge of warm fronts.

In this case, a spell of very cold sub-zero snowy weather was coming to an end. The cirrostratus is shown at the top of the photograph. It is white and translucent. The area of the sun is clearly visible.

Below the cirrostratus, the altostratus is a fairly uniform bluey grey colour and opaque. Below that are distinctive thin bands of stratus.

The front gradually moved into the area during the evening bringing periods of light snow, sleet and eventually rain.

Nimbostratus

Nimbostratus is the thickest and darkest cloud of the stratus family. It is particularly associated with the 'active' part of warm fronts, i.e. where moderate or even heavy rain is falling. The photograph opposite was taken from the Farne Isles in Northumberland, looking back towards the mainland, on a very miserable wet afternoon in July 2002. In the distance, the brighter skies are associate with higher level, thinner altostratus.

Stratus undulatus

This picture (taken at Alnmouth, Northumberland, July 2002) shows lower level stratus in the foreground. Its undulating lower surface classifies it as 'undulatus'. Towards the horizon a pallid sun can be 'seen' shining through a thin higher layer of altostratus. Such skies are typical of an approaching warm front.

The undulation is caused by undulating airflow: where the air is rising slightly the air cools below its dew point and stratus clouds are present; where there are clearer slots, the air is descending and warming and the cloud evaporates.

Stratocumulus

Stratocumulus is really a hybrid of stratus and cumulus. It occurs where there is very slight convection within a narrow vertical range. The result is that the cloud is not as flat as stratus, but lacks the vertical growth of cumulus. It can occur in a variety of thicknesses from thin sheets with lots of gaps (as opposite) to continuous, thick, grey layers which block out the sun. In the case opposite, in places the stratocumulus was thin enough to let the sun shine through it, so could more properly be classified as 'translucidus'.

If there are only limited areas of the cloud where it is thin enough for the sun to be seen then the cloud is classified as. In such cases the weather is still likely to be fairly bright, as is the case here at the mouth of the Tweed estuary.

In some cases stratocumulus can be of the much thicker 'opacus' type, through which the sun cannot be seen. This results in dismal grey weather, which may be accompanied by light rain or drizzle.

In the lowermost photograph the stratocumulus cloud nearest the camera does show some breaks, but over the sea it is much thicker. The arch like shape of the cloud is unusual. Arch shaped clouds are more common ahead of heavy thunderstorms. In this case, the cloud looks particularly dark because it was quite late in the evening In the distance the Farne islands can be seen. Stratocumulus typically forms over the sea areas around the British Isle and is blown onshore to give us grey weather. Despite its threatening look, not a drop of rain fell from the cloud opposite!