Altocumulus is a type of cloud which belongs to the group of clouds found at medium height, between 8,000 and 18,000 feet (around 2,500 to 5,500 meters). Its name is made up of the word alto, which is used to indicate medium height clouds, and cumulus, which means heap. This type of cloud is formed mainly from water droplets, and consists of patches or layers made up of flattened blobs or flakes. While it does not usually indicate anything more than light rain, if anything, altocumulus appearing on a warm morning and increasing may herald a thunderstorm later in the day.
Clouds are formed when air rises and cools, and the water vapor contained in the air condenses to form droplets. Three main groups of clouds exist, classed according to the heights at which they form into high, medium and low clouds. Altocumulus and altostratus make up the medium height group. Both are formed when a large mass of air rises over a hill or is lifted up when it meets a weather front, causing a wide area of condensation at medium height. Although the clouds are mostly made up of droplets of water, when the temperature becomes colder they can consist of ice crystals.
As well as being classed by height, the main cloud types may be further subdivided according to their shapes. Altocumulus lenticularis is the term used to describe altocumulus clouds when their shape is round and smooth, similar to a lens. Due to their saucer-like forms, it is possible that these clouds are occasionally mistaken for UFOs. They often arise on the sheltered sides of hills and may combine to form stacks or rows of saucers.
Altocumulus castellanus is another subtype. It occurs when the upper atmosphere is unstable, and the cloud takes on a turreted shape resembling a castle. When these clouds appear around the middle of the day they often indicate approaching thunderstorms. A different subtype known as altocumulus floccus is sometimes created when castellanus clouds break up.
Floccus clouds look like small tufts with rounded tops and fraying bases, which may indicate trails of rain or snow. This rain or snow, which does not reach ground level but trails from the bottom of the clouds, is known as virga. As well as taking on a number of different shapes, altocumulus clouds may be arranged in a variety of different patterns in the sky. They might consist of a sheet of tiny clouds which are quite close together, or may be distributed in lines or waves, which may be parallel to one another.