The term Altocumulus is a combination of two Latin words "Alto," a derivation of "Atlas", meaning "middle" and "cumulus," meaning "mass" or "heap". As the name suggests, altocumulus clouds are middle range clouds that form at an altitude of 6500 to 20000 feet (2000 to 7000 meters) above sea level. They are one of the ten fundamental cloud types or genera.
Altocumulus clouds are white or gray in color and form in layers or patches, broken into bands, blobs, rolls, or waves, or as a globular mass. They appear in groups, around one kilometer thick. Sometimes, they look like balls of cotton stuck into the blue background of the sky, and at other times, they form in rows.
Altocumulus clouds (abbreviated "Ac") are identified by the following species and variety, based on their size, shape, or form of the elements.
Ac Castellanus, which appear like pinnacles on a wall
Ac Floccus, which appear like little heaps,
Ac Stratiformis, which takes the shape of bumpy horizontal layers.
Ac Lenticularis, which have an especially dark and sometimes frightening appearance. Many people mistake this type for Unidentified Flying Objects (UFO's)
Ac Opacus or lacnuous, which obstructs sun and moon light,
Ac Translucidus, which is thin enough to be translucent
Ac Duplicatus, which occur in several layers,
Ac Opacus, which obscures sun or moonlight,
Ac Perlucidus, or an extensive patch, sheet or layer, with distinct but sometimes very small spaces between the elements that allow the Sun, the Moon, the blue of the sky or overlying clouds to be seen
Ac Radiatus, or parallel bands which, due to perspective, appear to meet at the horizon
Ac Undulates, including the Kelvin-Helmoholtz waves that form when there are two parallel layers of air that usually move at different speeds and in opposite directions.
Ac Nammatus, or rounded clouds that appear to hang on the underside of other clouds. They may or may not merge.
Altocumulus clouds are almost similar in appearance to the cirrocumulus clouds. However, altocumulus clouds are shaded and appear dark; whereas the cirrocumulus clouds are not shaded. Again, since the altocumulus clouds appear lower than cirrocumulus clouds, they appear larger. The altocumulus clouds also have some similarly to stratocumulus clouds, but appear smaller.
HOW THEY FORM
As the sun heats the earth, bubbles of hot air called thermals rise upward from the warm surface. These thermals of air parcels undergo convention. In other words, they continue to rise as long as the air within the parcel is warmer than the surrounding air. When the air inside the parcel mixes with the surrounding air and dilutes itself, it gradually looses the heat and some of its buoyancy. "Cold Front" is the name given to the transition zone where the cold air mass replaces a warmer air. When a cold front appears, temperatures suddenly drop by 15 degrees. The cool air, which now dominates the air parcel, condenses the moisture and produces clouds, and later induces precipitation.
Altocumulus clouds usually form by convection in an unstable layer aloft, which may result from the gradual lifting of air in advance of a cold front. In these layers, air currents undulate gently, like waves on the sea. As a wave rises, water vapors condense, and the clouds form. Water evaporates in the wave troughs, and as such the clouds become thinner or the sky remains clear. This is why altocumulus clouds sometimes form in patches or bands.
Altocumulus clouds consist of super cooled water at minus 10C. In the higher altitudes, ice crystals are also present.
HOW THEY AFFECT THE WEATHER
Altocumulus clouds do not usually produce rain, but might indicate a weather change within a day or so. The castellanus and/or floccus type of Altocumulus clouds indicate a high risk for afternoon thunderstorms when observed on a warm and humid summer morning. The lenticularis type of Altocumulus indicates weather deterioration within the next 12 to 36 hours. When the Altocumulus clouds show ripples and waves (undulate) the indication is that the weather will become worse within the next 12 hours.
HOW THEY DISSIPATE
Once formed, altocumulus clouds dissipate in four ways:
1. Subsidence or descent to a lower level when they transform to other cloud types,
2. Radiative heating, or the process by which temperature increases due to an excess of absorbed radiation over emitted radiation. When this happens, the clouds become partially fibrous and dissipate.
3. Entrainment of dry air, which again cause the cloud particles to dissipate
4. Precipitation, which takes place when the water vapor becomes dense, or the air cooler, and the clouds fall down as rain or snow. This happens rarely with Altocumulus clouds.