Mid-level clouds

Nimbostratus clouds

• Height of base: 2,000 - 10,000 ft
• Shape: Bands or areas of individual cells
• Latin: nimbus - rainy cloud; stratus - flattened or spread out
• Precipitation: Continuous rain or snow likely

Nimbostratus clouds are dark, grey, featureless layers of cloud, thick enough to block out the Sun. Producing persistent rain, these clouds are often associated with frontal systems provided by mid-latitude cyclones. These are probably the least picturesque of all the main cloud types.

How do nimbostratus clouds form?

Nimbostratus clouds form through the deepening and thickening of an altostratus cloud, often along warm or occluded fronts. These clouds extend through the lower and mid-layers of the troposphere bringing rain to the surface below.

What weather is associated with nimbostratus clouds?

These mid-level clouds are often accompanied by continuous moderate rain or snow and appear to cover most of the sky. Nimbostratus will often bring precipitation which may last for several hours until the associated front passes over.

If there is hail, thunder or lightning present it is a cumulonimbus cloud rather than nimbostratus.

How do we categorise nimbostratus clouds?

Nimbostratus clouds are featureless, very dense and have little characterisation, so are not classified into 'species' like other cloud types. They are similar to altostratus in this way.

Altostratus clouds

• Height of base: 6,500 - 20,000 ft
• Shape: Layered and featureless
• Latin: altum - height; stratus - flattened or spread out
• Precipitation: None

Altostratus are large mid-level sheets of thin cloud. Usually composed of a mixture of water droplets and ice crystals, they are thin enough in parts to allow you to see the Sun weakly through the cloud. They are often spread over a very large area and are typically featureless.

How do altostratus clouds form?

Altostratus layers are often composed of both water and ice and usually form when a layer of cirrostratus descends from a higher level. The Sun often cannot cast shadows when shining through altostratus clouds. These layers can sometimes contribute to the formation of optical effects such as coronas and iridescence.

What weather is associated with altostratus clouds?

Altostratus clouds often form ahead of a warm or occluded front. As the front passes, the altostratus layer deepens and bulks out to become nimbostratus, which produces rain or snow. As a result, sighting it can usually indicate a change in the weather is on the way.

How do we categorise altostratus clouds?

Altostratus clouds are featureless with little character, so are not classified into 'species' like other cloud types. They are similar to nimbostratus in this way. They do have many pattern-based varieties though, such as undulatus, radiatus and duplicatus, and thickness based varieties; translucidus and opacus.

Altocumulus clouds

• Height of base: 7,000 - 18,000 ft
• Shape: Bands or areas of individual cells
• Latin: altum - height; cumulus - heap
• Precipitation: None on its own

Altocumulus clouds are small mid-level layers or patches of clouds, called cloudlets, which most commonly exist in the shape of rounded clumps. There are many varieties of altocumulus, however, meaning they can appear in a range of shapes. Altocumulus are made up of a mix of ice and water, giving them a slightly more ethereal appearance than the big and fluffy lower level cumulus.

How do altocumulus clouds form?

Altocumulus clouds can form in several ways, such as; 

• Formation through the breakup of altostratus
• The lifting of moist air pockets which are cooled by gentle turbulence 
• Mountainous terrain producing atmospheric waves from which clouds can form. 

The presence of shading can help tell the difference between altocumulus and cirrocumulus. Cirrocumulus clouds are white and tiny, but altocumulus clouds can be white or grey with shaded sides.

What weather is associated with altocumulus clouds?

Mostly found in settled weather, altocumulus clouds are usually composed of droplets, but may also contain ice crystals. Precipitation from these clouds is rare, but even if rain does fall it doesn't reach the ground. This precipitation can be seen in the form of virga, where the rain is re-evaporated before reaching the surface.

How do we categorise altocumulus clouds?

Altocumulus clouds are one of the most diverse cloud types and have many different 'species'.

• Altocumulus stratiformis - The most common type of altocumulus, looking like flat-bottomed puffy clouds packed tightly together but separated by small rivers of sky. These can sometimes extend over the whole sky but are more common in smaller patches.
• Altocumulus lenticularis - One of the most spectacular cloud types, altocumulus lenticularis (also known as lenticular clouds) are lens-shaped clouds that form over hilly areas. Sometimes referred to as 'spaceship clouds' they often resemble the shape of a UFO. 
• Altocumulus castellanus - An indicator of instability, altocumulus castellanus towers can lead to the formation of cumulonimbus thunderstorms. These are more puffy looking and are taller than they are wide, making them stand out from other altocumulus varieties.
• Altocumulus floccus - Often spotted alongside altocumulus castellanus, altocumulus floccus is made up of slightly smaller, more ragged cloudlets. These are often found with virga clouds hanging below. 

What supplementary features are associated with altocumulus clouds?

Being a highly varied cloud type there are many features often associated with altocumulus. As previously mentioned, virga is known to regularly hang from the bases of altocumulus clouds, making for a jellyfish-like appearance. Taking this up a level, a fallstreak hole looks like a hole has been punched right through an altocumulus stratiformis layer.

Altocumulus castellanus

• Height of base:  (7,000 - 18,000 ft).
• Shape: A collection of small individual clouds, sometimes with “castle” towers proceeding from the top.
• Latin:  altum - height; cumulus – heap; castellanus = like a castle
• Precipitation:  Usually a few large droplets of rain, often evaporating before they reach the ground.

Altocumulus Castellanus clouds are a subset of Altocumulus clouds take their name from a resemblance to the turrets of castles and are often a warning of thunderstorms.

How do Castellanus clouds form?

Like other cumulus clouds, Castellanus clouds are caused by unstable air heated from below rising rapidly, causing water droplets to condense. The difference is that, whereas cumulus or cumulonimbus clouds are triggered by instability near the surface and heat from the sea or the ground, Altocumulus Castellanus clouds occur when the instability only starts much higher up. Although the cloud shapes look small to the eye, this is only because we see them from a great distance as they are so high up.

What weather is associated with Castellanus clouds?

Castellanus clouds are associated with lightning, often jumping from cloud to cloud without getting anywhere near the ground. Seeing Castellanus clouds is often a sign that Cumulonimbus clouds are on their way, with their associated heavy showers, strong gusty winds as well as thunder and lightning.