Altocumulus clouds

What are altocumulus clouds?

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 a number of ways, such as; formation through the breakup of altostratus, the lifting of moist air pockets which are cooled by gentle turbulence and 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 cloudlets 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 are 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 are 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.

https://www.metoffice.gov.uk