The sky is a mix of blue and gray. Is it partly sunny? Or mostly cloudy?
The terms can seem interchangeable; instead a litmus test to the meteorological version of "is the glass half full/half empty" argument.
But did you know the National Weather Service has specific definitions on each term used to describe cloud cover in their observations and forecasts?
And in their world, a mostly cloudy and partly sunny sky is not quite the same until the sun sets, and then they are.
The NWS uses a certain percentage of cloud cover to denote a sky condition, be it currently observed or in the forecast. But the terms are tweaked depending on day or night when they can or can not get away with using "sunny" words.
So during the day, a "partly sunny" sky is indeed sunnier than a "mostly cloudy" sky, which psychologically jives as well. "Partly sunny" seems a bit less gloomy than "mostly cloudy," even if you think about it from an English perspective where they should technically mean the same thing. If the pizza is "mostly" gone, there is still partly pizza left?
But at night, partly sunny morphs into mostly cloudy. That means if you're looking at a sky covered in 60% clouds, it's partly sunny until the sun sets, and then it's mostly cloudy. No fair! (Perhaps "Partly starry" was in order?)
Technically, along the same quirk, a partly cloudy night is less cloudy than a partly sunny day (Imagine a day and night that is 45% cloudy and reference the chart above.)
There are some exceptions to the way the NWS handles sky conditions in their forecasts. For example, the NWS won't usually use any cloud cover verbiage in their forecasts if the probability of precipitation is 60% or higher because…the cloud cover is kind of a given.
For example, a day with a 50% chance of rain might read "Cloudy with scattered showers," but if there is a 70% chance of showers, the forecast will read "showers" or "rain." (No "partly dry" here…)