What's the difference between 'Partly Sunny' and 'Mostly Cloudy'?

Partly sunny? Or Mostly cloudy? Sounds like the can be interchangeable but the National Weather Service has specific definitions on each term used to describe cloud cover.

Cool, cloudy weather as we head into the weekend - CBS Texas

What is the difference between “partly sunny” and “partly cloudy” when used in weather forecasts?

Why do people say partly cloudy and partly sunny like they're not the same thing? - Quora

Considering Clouds (and Slicing Pie) Learning Weather at Penn State Meteorology

Why do people say partly cloudy and partly sunny like they're not the same thing? - Quora

Every time I look at the weather forecast.. : r/AdviceAnimals

Why do people say partly cloudy and partly sunny like they're not the same thing? - Quora

Weather Terms You've Heard, But Probably Don't Know What They Mean

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