Cirrocumulus (Cc) clouds are a high-altitude cloud, characterized by their thin, white appearance. They are one of the three main cloud genera found in the upper troposphere, alongside cirrus and cirrostratus, and are composed almost entirely of minute ice crystals. Their presence indicates a dynamic process where horizontal layers of high clouds are disrupted.
Identifying Cirrocumulus Clouds
The defining feature of cirrocumulus clouds is their appearance as small, white patches or rounded tufts, known as cloudlets, often organized in extensive sheets or parallel rows. This distinctive, rippling pattern is frequently described as a “mackerel sky,” resembling the scales of a fish. To be classified as cirrocumulus, the individual cloud elements must appear very small, subtending an angle of less than one degree of arc. Cirrocumulus clouds are always bright white and translucent, which distinguishes them from lower, similar-looking altocumulus clouds that often show shading.
The Atmospheric Conditions Required
Cirrocumulus formation occurs in the high troposphere, at altitudes between 20,000 and 40,000 feet (6 to 12 kilometers). The atmosphere at this elevation is extremely cold, with temperatures often dropping below -20°C, ensuring the cloud composition is predominantly ice crystals. Although the air mass is generally dry, moisture must be present, often sourced from an existing cirrus or cirrostratus layer. This environment allows water vapor to transition directly into ice (deposition), or causes any remaining supercooled water droplets to freeze rapidly.
The Physical Mechanism of Formation
The transformation into cirrocumulus begins when a pre-existing, smooth layer of cirrus or cirrostratus cloud encounters localized uplift and turbulent airflow. This disruption is caused by wind shear—a difference in wind speed or direction between adjacent air layers in the upper atmosphere. The resulting friction initiates atmospheric waves, creating a series of horizontal rolls in the cloud layer.
As air rises within the crest of these waves, it experiences adiabatic cooling, encouraging the growth of ice crystals. Conversely, air sinking into the trough of the wave undergoes adiabatic warming, causing the ice crystals to sublimate or evaporate. This process breaks the continuous cloud sheet into the characteristic patchy, ripple-like cloudlets.
What They Indicate About Weather
In the short term, cirrocumulus clouds are associated with fair weather because they are too high to produce precipitation that reaches the ground. However, these clouds are indicators of increasing moisture and instability in the upper atmosphere. When observed alongside increasing amounts of cirrus and cirrostratus, cirrocumulus can signal the approach of a frontal system, such as a warm front. This suggests a potential change in weather, often with rain or snow arriving within the next 12 to 24 hours.