Altocumulus clouds are a middle-altitude cloud genus frequently observed across the globe. They occupy the mid-level layer of the troposphere. Classified as stratocumuliform clouds, they display characteristics of both layered and heaped structures. Altocumulus clouds are significant because they often indicate changes in atmospheric conditions, providing clues about upcoming weather patterns.
Defining Physical Characteristics
Altocumulus clouds appear as white or gray patches, layers, or rolls, often displaying a puffy, globular texture. The individual cloud elements are noticeably larger than those found in the higher Cirrocumulus clouds, but they are smaller than the elements in the lower Stratocumulus clouds. A simple way to judge their size is that the individual rounded masses are about the width of your thumb when held at arm’s length.
A distinguishing feature of Altocumulus is that they frequently exhibit shading or shadows on their undersides, a characteristic absent in the whiter, higher Cirrocumulus. The cloud composition consists mainly of water droplets, but in colder parts of the mid-level atmosphere, they can also contain supercooled water droplets or ice crystals. This mix of ice and water gives them a somewhat ethereal appearance compared to denser, low-level clouds.
How Altocumulus Clouds Form
The formation of Altocumulus clouds requires the lifting and cooling of moist air within the middle layer of the troposphere. This lifting can occur through various mechanisms, such as the gentle upward movement of air ahead of a weather front or the forced ascent of air over mountainous terrain. As the moist air rises, it expands and cools adiabatically.
Once the air cools to its dew point, the water vapor condenses into the small water droplets or ice crystals that make up the cloud. The characteristic patchy or rolling appearance results from mild atmospheric instability at that altitude, which creates shallow convection cells. Within this mid-level layer, air parcels rise in the lighter, cloudy patches and sink in the darker, clear spaces, leading to the distinct separation of the cloud elements.
Main Varieties and Weather Significance
Altocumulus clouds are diverse and classified into several distinct species, each indicating different atmospheric motions. One of the most common is Altocumulus stratiformis, which appears as relatively flat sheets or layers that can cover a large part of the sky. These layers are often broken into small, flattened patches or rolls, sometimes resembling fish scales, a pattern known as “mackerel sky.”
Altocumulus lenticularis is another notable species, characterized by its smooth, lens-like or almond shape, often forming downwind of mountains. These clouds are created by standing waves in the atmosphere and can be mistaken for flying objects due to their unusual form. A third species is Altocumulus castellanus, which features small, turret-like towers rising from a common base.
The appearance of stratiformis often suggests settled, fair weather, but other varieties carry significant meaning for forecasting. Altocumulus castellanus is a key indicator, as its turreted structure shows instability and convection occurring in the middle atmosphere. This instability can signal the potential development of thunderstorms later in the day, making it one of the “warning clouds” noted in aviation. While Altocumulus clouds rarely produce precipitation that reaches the ground, their thickening and lowering can precede the arrival of a frontal system that will bring rain or snow.