Mostly Cloudy or Partly Sunny?

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”.

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