What’s the difference between Partly Cloudy and Partly Sunny?
I found a great resource: the National Weather Service Glossary
The NWS defines Partly Cloudy as “Between 3/8 and 5/8 of the sky is covered by clouds.” And Partly Sunny, “Between 3/8 and 5/8 of the sky is covered by clouds.” But here’s the trick, “The term “Partly Sunny” is used only during daylight hours.”
Mostly Sunny is “When the 1/8th to 2/8ths of the sky is covered by with opaque (not transparent) clouds same as Mostly Clear, except only applicable during daylight hours.” While Mostly Cloudy is “When the 6/8th to 7/8ths of the sky is covered by with opaque (not transparent) clouds. Same as Considerable Cloudiness.”
It sounds like the progression works as follows:
Mostly Clear or Mostly Sunny: 1/8th to 2/8th clouds
Partly Cloudy or Partly Sunny: 3/8th to 5/8th clouds
Mostly Cloudy: 6/8th to 7/8th
Cloudy: 7/8th or more