From Cumulus to Cirrus: Exploring the Diversity of the Sky

5. Altocumulus: The Mackerel Sky

Usually developing between 6,500 and 20,000 feet, altocumulus clouds occupy the middle levels of the troposphere. Because their resemblance to fish scales when scattered throughout the heavens, these clouds are sometimes referred to as a "mackerel sky". Usually structured in parallel bands or waves, altocumulus clouds—which consist of water droplets and occasionally ice crystals—appear as spherical masses or rolls that grab the sight and imagination of sky watchers. These clouds can be really good markers of forthcoming changes in the temperature. The old weather lore "mackerel scales and mares' tails make tall ships carry low sails" speaks of the mix of altocumulus and cirrus clouds as a portent of approaching storms. Often appearing 12 to 24 hours before a front arrives, this proverb emphasizes the ability of altocumulus clouds to precede more major weather events, however not always true. Each of the several subtypes of altocumulus clouds has special qualities. Often signaling atmospheric instability and the possibility for thunderstorms later in the day, Altocumulus castellanus, for instance, resembles little castles or turrets rising from a common foundation. Because of their smooth, saucer-like look, altocumulus lenticularis—lens-shaped clouds—often go mistaken for UFOs. These clouds develop in the atmosphere as moist air passes over mountains or hills producing standing wave patterns. Particularly during sunrise or sunset when they capture the golden light, altocumulus clouds can produce beautiful sky sceneries that accentuate colors and patterns over the heavens. Their complex reflections and refractions of light often produce amazing visual effects, including iridescence—a rainbow-like play of hues along the cloud margins.