A “mackerel sky” is an atmospheric display characterized by a broad expanse of the sky filled with small, puffy clouds. This distinctive, textured pattern appears across large areas. Observing this phenomenon has traditionally been a way to anticipate upcoming changes in the local weather, offering insight into the complex dynamics occurring high above the earth’s surface.
Visual Characteristics and Naming
The appearance of a mackerel sky is defined by a series of small, rounded, white cloud patches arranged in a rippled, undulating pattern. These formations often appear in extensive rows or groups. The overall effect is highly textured, resembling the fine, regularly spaced scales on a fish’s back, which is the direct origin of the phenomenon’s common name.
The name specifically references the barred or spotted pattern found on the skin of the common mackerel fish. This visual similarity has also led to the alternative, though less common, moniker of “buttermilk sky,” describing the clouds’ curdled appearance. The cloud patches often have a delicate, semi-translucent quality, especially when viewed at high altitudes.
Atmospheric Conditions for Formation
The mackerel sky pattern is created by two main types of clouds: the mid-level altocumulus and the high-level cirrocumulus clouds. Cirrocumulus clouds form above 6,000 meters, where temperatures are below freezing, meaning they are primarily composed of tiny ice crystals. These high clouds are thin because limited moisture is available at that height.
Altocumulus clouds exist between 2,000 and 7,000 meters and are composed of supercooled water droplets. The rippled or wave-like undulatus pattern is a product of atmospheric instability, often caused by wind shear. This occurs when layers of air move at different speeds or directions, generating atmospheric waves.
These waves cause the air to rise and fall in a regular sequence, resulting in the uniform spacing of the clouds. Moist air lifted by the wave crests cools and condenses to form the cloud patches, while air sinking in the wave troughs causes dissipation. The formation of these clouds indicates that moisture is being lifted high into the atmosphere, often by a distant convergent zone or frontal system.
The Traditional Weather Significance
Historically, the appearance of a mackerel sky has been linked to forecasting imminent changes in weather. This predictive association is captured in the famous saying, “Mackerel sky, not twenty-four hours dry.” Another piece of folklore, particularly among sailors, states, “Mares’ tails and mackerel scales make lofty ships to carry low sails.” The “mares’ tails” refer to wispy cirrus clouds that often accompany the formation.
This traditional wisdom holds a strong meteorological basis, as the clouds often signal the distant presence of an approaching warm front or low-pressure system. The front pushes warmer, moist air higher into the atmosphere, creating the cirrocumulus clouds far in advance. If the clouds progressively thicken and descend into lower, denser layers, it suggests the front has moved closer, meaning precipitation is likely to arrive within the next 6 to 12 hours.