A good question came in from a friend today about what is the difference between Mostly Cloudy and Partly Sunny. While I had a general sense that it had to do with cloud percentages, I didn’t actually know the details. 10 minutes on google, and I got to the Weather Service Operational Manual (WSOM) Chapter C-11, Zone and Local Forecasts – section 8.6. In a more readable format:
| Term | Expected Predominant or Average Sky Condition |
|---|---|
| Cloudy | 9/10 to 10/10 opaque clouds |
| Mostly Cloudy or Considerable Cloudiness | 7/10 to 8/10 opaque clouds |
| Partly Cloudy or Partly Sunny | 3/10 to 6/10 opaque clouds |
| Mostly Clear or Mostly Sunny | A Few Clouds and 1/10 to 3/10 opaque clouds |
| Clear or Sunny | 0/10 to 1/10 opaque clouds |
Opaque means you can’t see the sun, moon, or stars through them (so wispy clouds don’t count). Terms like “Fair” are specifically deprecated (though defined), and there is even a list of words that “are viewed as hedge terms and should be used sparingly and with great discretion”.
It's always more complicated 