What is the difference between “partly cloudy” and “partly sunny” in weather forecasts?
Your question comes up occasionally, so let’s address it again. First of all, “partly sunny” is a daytime term. It cannot be used at night,
Four terms describe the amount of the sky covered by clouds: Clear, used when the sky is clear or covered by up to two-tenths of clouds; partly cloudy, when cloudiness ranges from three-tenths to six-tenths of coverage; mostly cloudy, clouds covering seven-tenths to nine-tenths of the sky; cloudy, when cloudiness is nearly or fully complete. In addition, “overcast” refers to clouds covering all of the sky.
Sunny, mostly sunny, partly sunny and cloudy (sunless) have comparable definitions for daytime situations.
The use of all these terms is loose, but those are the precise definitions.