'Mackerel sky' refers to a state of sky with extensive clouds that look like fish scales in a bright day. These clouds are small and white, and usually line themselves up in groups in a regular pattern. They resemble the small ripples created by a gentle breeze on water surface. In meteorology, they are called 'cirrocumulus' clouds.

'Cirrocumulus clouds' are high clouds, generally with a cloud base at above 6000 metres where the temperatures are several tens of degrees Celsius below zero. They are composed of tiny ice crystals and are thin and translucent. Since there is not much water vapour at that altitude, the growth of ice crystals in the cloud is limited and the crystals can remain suspended atop some ascending air. The ice crystals may occasionally fall from the clouds, but they tend to evaporate on the way down and seldom reach the ground.

The occurrence of cirrocumulus clouds indicates the distant presence of a convergent zone such as a pressure tough or a cyclone in lower levels; or a frontal zone where cold air meets moist warm air. Vigorous convection and lifting motion bring water vapour up in the sky where the water vapour sublimes into ice crystals, forming the high clouds. Under the influence of atmospheric waves which are often present at high altitudes, the clouds line themselves up and move with the upper level winds, forming the cirrocumulus clouds having a fish-scale appearance.

'Mackerel sky, not twenty-four hours dry' describes the deterioration of weather after the appearance of cirrocumulus clouds. Although it may be bright at the beginning, the weather will deteriorate in half a day's time as the cyclone or front approaches. As they pick up more moisture, the cirrocumulus clouds thicken and extend downward to become low-level clouds which bear rain.

Observation of cloud change is an important tool in weather forecasting. The folklore of 'Mackerel sky, not twenty-four hours dry' reflects the experience and wisdom passed down by people over the years.