Mackerel sky is a term used to describe a sky that is filled with rows of small, white or grey clouds. These clouds often resemble the scales on the skin of a mackerel fish hence the name. However, there are several other terms that can be used interchangeably with the term "mackerel sky". Some of these synonyms include "buttermilk sky", "sheepback sky", "jigsaw sky", and "puzzle sky". These terms are used to describe similar cloud formations and can be used to enhance the description of the sky's appearance. It's essential to note that these terms are weather-related and can help predict upcoming conditions.