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Information about Altocumulus Clouds

Altocumulus clouds are middle-level clouds that form at a height of around 6,500 to 20,000 feet above the sea level. There are several species and varieties of altocumulus clouds.

Did You Know?

The name Altocumulus (Ac) comes from the Latin words altus meaning high and cumulus meaning mass or heap.

Altocumulus clouds are small round patches of clouds with convective characteristics. They come in various shapes and their degrees of transparency vary. They generally look like rounded masses and sometimes like rows of clouds. They are either white or gray in color. They have a thickness of around one kilometer and are mostly seen in a settled weather. They normally constitute water droplets and sometimes ice crystals. They are formed when water droplets from the sea or ground are carried up by the wind to higher atmosphere levels. Cool air at the higher altitudes causes condensation of water droplets in the air, and the clouds are formed.

Altocumulus clouds may tell about unstable weather conditions in the morning and high convection later in the day. They are easily seen in the sky, and can be seen in any part of the world. On the basis of their shape and internal structure, these clouds are classified into various species. On the basis of their degree of opacity, they are classified into certain varieties.


Altocumulus Stratiformis (Ac str)

Altocumulus Stratiformis

These clouds appear like flat sheets or layers or like lumpy jet trails. They are the thinnest of all the altocumulus clouds. These clouds tell us that the weather is switching from good to unstable. After a period of bad weather, they are the last clouds seen in the sky. It’s quite common to see these clouds, and they can be seen all over the world.

  • The altocumulus stratiformis clouds are formed when stable winds at the ground level carry moist air to higher altitudes. If these winds are gentle, they allow the water particles to be carried beyond the lower altitude levels. After reaching a medium height, strong winds cause the water particles to form these clouds.
  • These clouds are formed at a height of 6,000 to 20,000 feet.
  • As they form at the higher levels, they are made up of ice crystals and hence, reflect more light. As a result, they are easily visible in the sky.
  • Altocumulus stratiformis are usually of three varieties: translucidus, perlucidus and opacus. The duplicatus, undulatus, radiatus, and lacunosus varieties are also seen with stratiformis.

Altocumulus Lenticularis (Ac len)

Altocumulus Lenticularis

These clouds are formed in the shape of a lens or an almond. They sometimes appear like piles of plates, flying saucers, a cake, or as giant lids above the mountains. These clouds bring light rain or snow. They are formed over high mountains in any part of the world.

  • The altocumulus lenticularis clouds are usually formed when moist air blows over the mountains. Winds carrying air surge up over one side of a mountain and the moisture in the air condenses to form clouds. As the wind comes down the other side of the mountain, it warms up. The water in the cloud converts back to water vapor because warm air can hold the vapor. This constant wind motion over the mountains leads to the formation of lenticular clouds.
  • They are formed at a height of 6500 to 16,500 feet.
  • The varieties duplicatus or undulatus are seen with the altocumulus lenticularis.

Altocumulus Castellanus (Ac cas)

Altocumulus Castellanus

These clouds resemble the turrets of castles. As they indicate instability in the middle layers of the atmosphere, they may indicate a thunderstorm. These clouds aren’t a threat to the safety of aircraft traveling through them. However, pilots experience instability when traveling through these clouds. They are seen in higher mountain ranges, over large inland lakes, during the spring and summer months, and when the wind at the ground level is not very strong.

  • The altocumulus castellanus clouds form because of atmospheric convection and are mid-level clouds. This happens due to air instability due to the slow rising of warm, moist air coming from the cold frontal system. Following this, clouds with surges or waves start developing from the main deck and condensation within air pockets leads to the castellanus effect.
  • They form at the heights of about 7,000 to 18,000 feet.
  • The variety radiatus is seen with castellanus. The variety lacunosus is also seen, but rarely.

Altocumulus Floccus (Ac flo)

Altocumulus Floccus

In Latin, the word floccus means lock of wool. These clouds look like puffs of cotton or wool. They have ragged bases and rounded heads. The colors of these clouds vary from white to a grayish opaque. The sky is not visible beyond the altocumulus floccus clouds, as they are opaque. They bring rain or snow. They can be seen anywhere in the world.

  • The altocumulus floccus clouds are normally formed when there is unstable mixing of warm and cold air.
  • The warmer the climate is at a specific place, the higher the cloud’s base.
  • These clouds are usually formed at about 6,500 to 13,000 feet.
  • The variety lacunosus is seen with altocumulus floccus clouds.


Altocumulus Radiatus

Altocumulus Radiatus

They appear as parallel bands of clouds. They usually have a main body from which wisps of the cloud radiate. The color of these clouds is either white or gray. These clouds are very common. They are mostly seen in the temperate zones. These clouds cause frosting on the wings and other parts of planes because they normally contain a large amount of moisture in the form of ice, water, or snow.

  • The altocumulus radiatus clouds are formed when air with moisture is forced to move to the upper atmosphere. Here, the cold air forces the liquid within the air to condense, forming particles that make the clouds.
  • They are formed at a height between 6,000 and 16,000 feet above sea level.
  • Clouds with tiny droplets of water or ice within them disperse light. This means that the clouds will appear dark. Clouds with larger droplets will allow more light to pass through. Hence they will look whiter than those with smaller droplets.

Altocumulus Perlucidus

Altocumulus Perlucidus

These clouds appear irregular and are commonly seen as lumps in the sky. There seem to be small gaps in these clouds. These clouds are commonly seen in the temperate zones. However, they are very volatile i.e. they form and dissipate soon. These clouds can cause mechanical problems in the airplanes that fly through them.

  • For the altocumulus perlucidus clouds to form, a mass of
    air at the ground level needs to have some moisture content. This could be through evaporation when the sun heats up the ground, or it could be when the air mass flows over a water body, accumulating water as it travels over it. These clouds are generally formed in the same way as other clouds. Cold air combining with moist air or a mountain causing air to move up due to conventional currents can lead to the formation of these clouds.
  • They form at a height between 6,000 and 16,000 feet.

Altocumulus Duplicatus

The altocumulus duplicatus clouds are formed in two or several layers lying one above the other. These clouds are not easily spotted. It is relatively easy to see the lower layer as it is of a darker shade than the upper ones. The top layer is not easily seen. However, at sunset, when the sun is the lowest in the sky, the layering of these clouds is more visible. They can be seen in all parts of the world.

  • The altocumulus duplicatus are formed by the accumulation of water droplets combining with warm air.
  • They are formed at a height of 6,500 feet and more, but not above 16,000 feet.

Altocumulus Undulatus

Altocumulus Undulatus

The altocumulus undulatus clouds appear as gently rolling waves or ripples of water in the sky. The undulating bases of these clouds are due to the eddies at their base, thus creating wave-like patterns. They are white or gray in color and are around 300 feet thick. The formation of these clouds indicates that there is no risk of storm or heavy rains. They bring light rain. They are visible all through the year.

  • The altocumulus undulatus clouds are formed just like any other clouds. The moisture content in air at the ground can be acquired on moving over a large water body, or due to the vapor of surface water.
  • They form at a height between 6,000 and 20,000 feet above the sea level.
  • They are made up of water, ice, or snow.

Altocumulus Translucidus

Altocumulus Translucidus

These clouds appear as a screen in the sky. They are flat and resemble a translucent sheet of glass across the sky. As they are translucent, they allow sunlight or moonlight to be visible from behind them. They do not indicate rainfall, but indicate precipitation in the next 10 to 20 hours from the time they are visible. These clouds are very common and are seen everywhere in the world.

  • The altocumulus translucidus clouds are formed by the transfer of heat between the warm air and water drops or ice crystals, depending on the height at which they are formed.
  • These clouds form at a height between 6,500 and 20,000 feet.
  • They are fairly common and clearly visible in the sky.

Altocumulus Lacunosus

Altocumulus Lacunosus

They appear as thin clouds with perforations in them. In Latin, lacunosus means ‘full of holes’. These clouds look like a loose honeycomb with rough holes between the clouds. Holes appear along the boundaries of these clouds. They are less common and are most visible during the summer season in certain countries. They are more often visible all through the year in countries that have a tropical climate. However, in some parts of the Western hemisphere, these clouds are rarely seen.

  • The altocumulus lacunosus clouds are formed due to the sinking pockets of air that exist and which result in the formation of holes in the clouds. The cloud fringes are formed by the air coming up between the holes in the clouds.
  • They form at a height of 6,500 to 13,000 feet.

Altocumulus Opacus

These clouds are thick and appear as a dark layer of clouds. The sun or moon cannot be seen through them. If these clouds are formed as a result of ice crystals, they may be hazardous to aircraft due to the ice accumulation on the aircraft as it flies through the cloud. They do not indicate storms or heavy rains. They can be seen often and are easily visible. These clouds are usually seen in the United Kingdom.

  • The altocumulus opacus clouds are formed as a consequence of water droplets coming in contact with warm air. If the temperature is low, warm air combines with ice crystals instead of water droplets.
  • These clouds are formed between 6,500 and 20,000 feet.

Altocumulus clouds are classified into two types.

Altocumulus Mamma

They appear like protrusions from the base of the clouds. They have a substantial amount of moisture and are formed very quickly. They are the most commonly seen clouds. They are volatile and can be seen in the temperate zones of the world.

  • In the formation of the altocumulus mamma clouds, the condensation of water within the clouds has to occur at an altitude of around 6,000 to 16,000 feet. Air that has little water in it would need lower temperatures to cool. So, the air would have to move higher to condense. Also temperatures are lower at higher altitudes.
  • They form at a height between 6,000 and 16,000 feet above sea level.

Altocumulus Virga

Altocumulus Virga

They look like wisps of clouds which come down from the main body with the base of the clouds towards the ground. The clouds can only bring some precipitation like rain or snow. However, mostly, the precipitation does not reach the ground, which leads to the formation of wisps. These clouds may pose a risk to pilots flying planes at this height. These clouds are commonly seen in places with moderate humidity. They are volatile.

  • Similar to other types of clouds, the formation of the altocumulus virga clouds is due to the condensation of air containing moisture. The rising of air can be due to a mountain that makes the air move up, by convectional currents, or by cold air fronts.
  • They form at the heights of 6,000 to 16,000 feet above sea level.
  • They can be seen in many temperate zones of the world, like the southern states of the US, and some areas in UK and Europe.

The different patterns and shapes of clouds are simply amazing. The various types of clouds and their formation are subjects of interest for many researchers. Several aspects of this interesting phenomenon are still not known.

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