Partly

When I woke up this morning, one of the first things I did was to check today's weather forecast. I learned that the prediction was for partly sunny skies. On other days, it has been forecasted to be partly cloudy.

What's the difference?!

So, I decided to do a little research and here are three explanations I found.

from wunderWIKI
Unfortunately, the weather terms Partly Cloudy and Partly Sunny are often confused, even by meteorologists. Partly Sunny means only part (less than half) of the time period will be sunny. In that same respect, Partly Cloudy means only part of the period will be cloudy. Since there is often confusion with these terms, however, it is best to assume that they are almost interchangeable.

Partly cloudy In a forecast, this means that less than half of the day will be cloudy. For a current condition, this means that less than half of the sky is covered by clouds.

Partly sunny In a forecast, this means that more than half of day will be cloudy. For a current condition, this means that more than half of the sky is covered by clouds.

from Diffen
Weather reporters often use the words partly sunny or partly cloudy skies on TV and radio. While this can get confusing, there is really no difference between partly sunny and partly cloudy as far as the meaning is concerned. There are, however, often different conditions when these phrases are used.

Even though they mean the same thing viz. the sun will be partially covered with clouds, newscasters use both phrases so that there is some variety in their vocabulary.

Since you cannot use "partly sunny" to describe weather conditions at night, weather reporters use the phrase "partly cloudy" for describing conditions at night and often use "partly sunny" for describing conditions during the day.

from Ken Kaye's Storm Center
What’s the difference between partly cloudy and partly sunny?

You see those phrases frequently in weather forecasts, and (as commenter KCK pointed out ) it's a bit confusing because they would seem to be the same thing.

Guess what?

They are the same thing, said meteorologist Andy Tingler of the National Weather Service in Miami.

“There’s no difference between partly cloudy and partly sunny,” he said.

Well, there is one minor difference, he added:

“We use partly sunny in the daytime and partly cloudy at night – because you can’t say partly sunny at night.”

By definition, the weather service says it’s partly cloudy when clouds cover 26 to 69 percent of the sky, Tingler said.

Interestingly, Florida, the Sunshine State, is partly cloudy more than any other state in the union, the weather service says.

This is all well and good, but it didn't help with today. There was no partly to describe our skies. No, the better descriptor for today was mostly. As is often the case here, it was mostly cloudy.