There is no difference between partly cloudy and partly sunny, according to the National Weather Service
Partly Cloudy Between 3/8 and 5/8 of the sky is covered by clouds. Partly Sunny Between 3/8 and 5/8 of the sky is covered by clouds. The term "Partly Sunny" is used only during daylight hours.
As explained above, “Partly sunny” and “partly cloudy” are both used to describe weather conditions with some clouds.
It is generally understood that “partly sunny” means there is some sunshine visible, while “partly cloudy” means mostly cloudy skies with some clear breaks.
Let’s examine which is more frequently used.
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Partly cloudy means that the sky has some cloudy parts and some not cloudy parts.
Expect a partly cloudy and mild day with a slight chance of isolated showers and afternoon temperatures in the mid to upper 60s.
High temperatures will reach the upper 70s and low 80s under partly cloudy skies.
But partly cloudy skies are also between 37% and 63% cloudy.
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During the day, the sky is partly sunny. During the night, we use partly cloudy. Mostly sunny means there is more sun than clouds.
After cold temperatures and snow showers over the weekend, we’re starting the workweek on a nicer note, with partly sunny skies.
Saturday will be partly sunny, with afternoon highs expected to reach almost 80.
Friday morning’s clouds will gradually clear to a partly sunny sky.