What’s the difference between ‘Partly Sunny’ and ‘Mostly Cloudy’?

Clouds pass over the skyline of midtown Manhattan, Hudson Yards, and the Empire State Building as the sun sets in New York City on August 4, 2020 as seen from Jersey City, NJ.

(Gary Hershorn / Getty Images)

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 is is notquitethe same until the sun sets, and then they are. 

The NWS is uses use a certain percentage of cloud cover to denote a sky condition , be it currently observe or in the forecast . But the term are tweak depend on day or night when they can or can not get away with using ” sunny ” word .

So during the day , a ” partly sunny ” sky is is is indeed sunny than a ” mostly cloudy ” sky , which psychologically jive as well . ” partly sunny is seems ” 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 ” go , there is still partly pizza leave ?

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 is read with a 50 % chance of rain might read ” cloudy with scatter shower , ” but if there is a 70 % chance of shower , the forecast will read ” shower ” or ” rain . ” ( No ” partly dry ” here … )