Partly Sunny skies occur when 3/8ths to 5/8ths of the sky is covered by clouds. The term Partly Sunny is used only during daylight hours, and is replaced by Partly Clear at night.
Partly Cloudy
Partly Cloudy skies occur when 3/8ths to 5/8ths of the sky is covered by clouds.
Mostly Sunny / Mostly Clear
Mostly Sunny skies occur when 1/8th to 2/8ths of the sky is covered by clouds. The term Mostly Sunny is only used during daylight hours, and is replaced with Mostly Clear at night.
Mostly Cloudy
Mostly Cloudy skies occur when 6/8ths to 7/8ths of the sky is covered by with clouds.