Altocumulus clouds (Ac) are common mid-level atmospheric formations that appear as white or gray patches, layers, or rolls. These clouds reside in the middle layer of the troposphere, typically between 6,500 and 20,000 feet above the ground. Their presence is a visual indicator of conditions at this altitude, and they often serve as an early signal for changes in the prevailing weather pattern.
Identifying Altocumulus Clouds
Identifying Altocumulus requires observing their altitude and the relative size of their individual elements. These formations are made up of water droplets and sometimes ice crystals, appearing as distinct, rounded masses or rolls with some gray shading. The classic visual test to confirm an Altocumulus cloud is to extend your arm toward the sky. If the apparent size of a single element is about the size of your thumbnail when held at arm’s length, you are likely looking at Altocumulus.
This rule distinguishes them from the higher Cirrocumulus (which appear much smaller) and the lower Stratocumulus (which appear much larger). Altocumulus often display an undulating or wavy pattern, sometimes called a “mackerel sky” because of its resemblance to fish scales. This patchy appearance results from stable air currents creating rippling patterns in the cloud layer.
General Weather Indications
The appearance of widespread Altocumulus clouds generally points to atmospheric instability and a transitional period in the weather. These clouds are created by convection reaching the mid-levels of the troposphere, but they rarely produce rain that reaches the ground themselves. A classic scenario involves a sheet of Altocumulus thickening and lowering throughout the day. This pattern often signals the slow approach of a low-pressure system or a warm front, which may bring precipitation within the next 12 to 24 hours. Conversely, Altocumulus seen during the morning of a clear, warm day can indicate that the air mass is relatively stable, leading to continued fair weather or clearing skies later in the afternoon. The old adage, “Mackerel sky, not long dry,” suggests that a patch-filled sky often precedes a period of rain.
Specific Altocumulus Variations and Predictions
Certain sub-species of Altocumulus carry much more specific weather predictions than the general formations.
Altocumulus Castellanus
Altocumulus Castellanus takes on a turreted or castle-like appearance with small, rising towers extending from a common base. These formations are a strong indicator of instability in the mid-levels of the atmosphere. The towers show that air is rising rapidly, and if that mid-level instability connects with convection near the surface, Altocumulus Castellanus is a reliable precursor to the development of thunderstorms or heavy showers later in the day.
Altocumulus Lenticularis
Another highly distinctive form is Altocumulus Lenticularis, recognized by its smooth, lens-shaped, or saucer-like appearance. These unique clouds form when moist air flows over a topographic barrier, like a mountain range, creating a standing atmospheric wave. Although they appear stationary, the clouds continually form on the upwind side and dissipate on the downwind side. Their presence signals powerful vertical air movements and strong winds aloft, posing a significant warning for aviation due to potential severe turbulence.