in my experience its more like: 0-5% - Sunny; 5-60% - Partly Cloudy; 60-100% - Cloudy;

I've not seen other descriptors, and to view it changing over time its just on the timeline as a sun, sun+cloud & cloud emoji's

It's not useful other than as a binary 'is it cloudy' which in Bay Area weather it is a meaningful difference between 30% and 60% coverage