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What is ACC in Aviation? (Altocumulus Castellanus)

Updated: February 14, 2024

Understanding Altocumulus Castellanus (ACC) in Aviation

Altocumulus Castellanus, commonly known as ACC, is a type of cloud formation that occurs at middle altitudes in the atmosphere. These clouds are characterized by their distinct castle-like towers or turrets, which give them a unique appearance. ACC clouds are often associated with unstable atmospheric conditions and can signal the potential for thunderstorms or other severe weather. In this article, we will delve deeper into the characteristics, formation, and significance of Altocumulus Castellanus in aviation.

The Formation of Altocumulus Castellanus

Altocumulus Castellanus clouds typically form between 6,500 to 20,000 feet above ground level, in the middle layer of the atmosphere known as the altostratus layer. These clouds are composed of water droplets and occasionally ice crystals, depending on the temperature at their altitude. The presence of ACC clouds indicates the presence of moisture and instability in the atmosphere, which can lead to the development of thunderstorms or other convective weather phenomena.

Altocumulus Castellanus clouds form when moist air rises and encounters a layer of instability in the atmosphere. This instability can be caused by factors such as temperature inversions, wind shear, or the presence of a frontal boundary. As the moist air rises, it cools and condenses, forming cloud droplets. Under the right conditions, these droplets can organize into the characteristic castle-like towers or turrets of ACC clouds.

The Significance of Altocumulus Castellanus in Aviation

Altocumulus Castellanus clouds play a significant role in aviation weather forecasting and flight planning. Pilots and meteorologists closely monitor the presence of ACC clouds as they can indicate the potential for severe weather, including thunderstorms, turbulence, and icing conditions. Understanding the formation and behavior of ACC clouds is crucial for pilots to make informed decisions and ensure the safety of their flights.

When pilots observe ACC clouds, it is important to exercise caution and be prepared for rapidly changing weather conditions. These clouds often precede the development of thunderstorms and can provide early warning signs of convective activity. Thunderstorms pose various hazards to aviation, including severe turbulence, strong updrafts and downdrafts, lightning, and heavy precipitation. Pilots should avoid flying near or through ACC clouds and closely monitor weather updates to determine the appropriate course of action.

Additionally, ACC clouds can also indicate the presence of wind shear, which is a sudden change in wind speed or direction over a short distance. Wind shear can have adverse effects on aircraft performance, particularly during takeoff and landing. Pilots should be aware of the potential for wind shear when ACC clouds are present and take the necessary precautions to ensure safe operations.


Altocumulus Castellanus (ACC) clouds are an important indicator of atmospheric instability and the potential for severe weather in aviation. Pilots and meteorologists rely on the presence and characteristics of ACC clouds to make informed decisions regarding flight planning and safety. By understanding the formation and significance of ACC clouds, pilots can navigate the skies more effectively and minimize the risks associated with convective weather phenomena.

For more information on Altocumulus Castellanus and its impact on aviation, you can visit the National Weather Service website, which provides detailed weather forecasts, aviation weather products, and other valuable resources for pilots and meteorologists.

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