What's the Difference Between 'Mostly Sunny' and 'Partly Cloudy'?
cpexecutive.com, Tiny Fox Lab

An okta is a unit of measurement that meteorologists use when they’re forecasting cloud conditions, equal to 1/8 of the sky. So the difference between “partly sunny” and “mostly cloudy” is about 1 to 4 oktas.Tiny Fox Lab Facts

Preview photo credit: cpexecutive.com
Source: https://www.mentalfloss.com/article/56820/whats-difference-between-mostly-sunny-and-partly-cloudy